Download the form N-400 here 

And then follow the instruction below to file your Citizenship Application

How clearly and carefully you prepare your paperwork can affect how your application is judged. It’s important to: Deal appropriately with inapplicable questions. If you know that a question on Form N-400 doesn’t fit your situation, write “N/A” (not applicable) rather than leaving the space blank. If you don’t feel certain how to answer a question, consult an attorney.

Be honest. Lying to USCIS can get you in bigger trouble than the problem you are lying about. Even if no one discovers the lie now, it can result in your citizenship being revoked later—even many years later. Again, if you are unsure how to deal with an issue, contact a lawyer.

Be consistent with your previous immigration applications. Check your files for other applications or paperwork that you have submitted to U.S. immigration authorities and double check that the information you are entering now matches what’s there -- unless, of course, the previous applications contained typos or minor errors. Enter the correct information on your N-400, but be ready to explain the previous error and provide evidence of the true situation.

Use extra pages if need be. In a few places on Form N-400, your response may need to be longer than the form has room for. If so, write “Please see attachment” in the appropriate space on the form and then attach a separate piece of paper. At the top of the attachment page, print your name, A-number, and the words “Attachment to Form N-400.” Then state which question(s) from which part(s) of the form you are answering.
Don't staple or otherwise puncture the barcode. This may interfere with USCIS's ability to process your application.

Line-by-Line Instructions for Form N-400

Now, let’s go through the form, question by question (but skipping instances where the questions are self-explanatory). Note that at the top of every page of the form, you must remember to fill in your A-number.

Part 1. Information About Your Eligibility
Check the box matching your citizenship eligibility basis. Most people check Box 1 (five years of permanent residence).
However, spouses of U.S. citizens can check Box 2 (if they have been married and living together for three years) and people in military service can check Box 3, stating that they qualify for an exception to the five-year rule. (Remember that you will need to provide proof that you fit in the stated category.) Refugees and asylees should check Box 1. Although they are permitted to credit some of their years as refugees or asylees toward the five-year requirement, they are not exempt from the five-year requirement itself.

Part 2. Information About You
Question 1. Your current legal name. Enter your full name. If your name has ever changed – perhaps due to adoption, marriage, or a court-ordered name change -- include legal documentation of the change.

Question 2. Your name exactly as it appears on your Permanent Resident Card. This question helps USCIS match your application to your green card, so copy your name exactly as it appears there, even if there are mistakes or your name has been or will be changed.

Question 3. If you have ever used other names, you will have a chance to enter them below. Here you should provide only alternate versions of your name (such as from past marriages) as well as names by which you have been commonly known. For example, if your legal name is Juozas but everyone calls you Joe and it appears on some of your personal records and documents, mention Joe on this part of the application. If, however, only your wife or a few friends call you Joe, then you don’t need to write it here. If unsure, simply enter the nickname, to be on the safe side.

Question 4. Name change. If you have been wanting to legally change your name, this may be your opportunity. You can legally change it without any extra court procedures by filling in your chosen new name on Form N-400; on one condition. Your swearing-in ceremony must be held in a courtroom, presided over by a judge, not by a USCIS officer. In some USCIS districts, ceremonies presided over by a judge are held only a few times per year if at all, so asking for a name change will either be impossible or will result in your waiting longer than other people to receive citizenship. Contact your local USCIS office and for details. If you are able to request a name change, the USCIS officer will have you fill out a form called a Petition for Name Change during your interview. Realize also that the law places limits on what you can change your name to

Question 5. U.S. Social Security Number. This is self-explanatory -- you most likely have a legal Social Security Number by now. Don't put down a false one!

Question 6 is simply your date of birth.

Question 7. You can find the date you became a permanent resident on your green card.
In Questions 10 and 11, you can advise USCIS as to whether you have any physical or mental disabilities, that is:
whether USCIS can do something in particular to make your interview easier for you to handle, such as providing wheelchair access or a sign language interpreter, and whether you are requesting a reduction or waiver of the English language and/or U.S. history and government exam requirements because of your disability.

It’s fine to say yes to both questions. If you say yes to the second one, however, you will need to have a doctor back up your request, by examining you and preparing USCIS Form N-648. (we are working on post about waiver)
There is actually a third issue regarding disability that Form N-400 doesn’t cover. Applicants whose conditions are particularly severe may, even after being approved for citizenship, be unable to understand or repeat the Oath of Allegiance that makes them a U.S. citizen. The applicant will not automatically be allowed to skip this oath, but someone must first request a separate waiver of the oath requirement on his or her behalf (assuming that the applicant cannot speak for him- or herself). USCIS suggests using the space under this question to request the oath waiver; or else doing so in the cover letter to the N-400 or in a separate statement.

Question 12 allows you to elect to take an age-related, easier version of the English language test that is part of the naturalization exam.

Part 3. Information to Contact You
Questions 1-5. These ask for your phone numbers and email address. In cases where it's appropriate, you can answer “none.” Or, you could use a friend or neighbor’s phone number -- USCIS is unlikely to actually call you. Be sure you provide some sort of workable contact information, just in case.

Part 4. Information About Your Residence
First, enter your current home address. Below that is a space to enter a different mailing address, if your home address is an insecure place at which to receive mail. Next, you will need to enter all your addresses for the past five years, from most recent to longest ago. One of the main purposes of this section is to make sure that you are not disqualified from applying for citizenship right now because you have spent too much time abroad.

Part 5. Information About Your Parents
Your answers to this question could result in your saving a lot of time and effort! If you happen to have parents who are U.S. citizens, there is a chance they transmitted their citizenship to you, automatically. Whether this occurred depends on various factors, such as your date of birth, how long your parents lived in the U.S., and whether one or both of them were citizens. If they did pass on their citizenship to you, you need not fill out Form N-400, but can claim your citizenship in other ways.(we are working on post about  "Acquiring or Deriving Citizenship Through Parents"  stay tune)

Part 6. Information for Criminal Records Check
Fill in this information honestly. If you do not believe you fit into any of the categories, choose the one that is closest. Under “Race,” people of Latin American or Hispanic background typically choose “White.”
If you have a criminal history, don’t try hiding the truth — USCIS will separately check your fingerprints against various law enforcement databases. If you are have reason to believe that USCIS will turn up negative information in its check of your records, by all means consult an immigration lawyer.

Part 7. Information About Your Employment and Schools You Attended
Here, you must give details about where you went to school or worked for the last five years. If you cannot remember something from years past, put in as much information as you can remember. If you have gone through periods of unemployment, unpaid work, self-employment, disability, retirement, or taking care of your home or children, list these too.
Include any time you spent working in the U.S. illegally before you got your green card (but make sure that it matches the information on your green card application forms).
What if you spent all of the past five years not working, but had a job before that? It's a good idea to list that job in Question 2.

Part 8. Time Outside the United States
You will need to prove that you spent the required minimum amount of time in the U.S. in the years leading up to filing your N-400 and that your visits outside the United States -- including to Canada and Mexico as well as any other countries -- did not last too long.
If you cannot determine the information, enter as much as you can. For example, some people may write in the space (or on an attachment page) something like, “I crossed the border into Canada to spend time with my father approximately once a month for the last six years. Most of my visits were two days long, except for visits at Chinese New Year, when I usually stayed for five days.” Or you can put "APPROX" in front of the date, to indicate that it's an approximation, or your best estimate of the date.

Part 9. Information About Your Marital History
This section raises questions for many applicants. For example, Question 8 is a source of confusion. It asks, “How many times has your current spouse been married?” The key is to understand that your current marriage counts in adding these up. So, for instance, if your spouse had been married twice before, you would answer “three” here.
Another common concern is the question about your spouse’s immigration status, particularly if your spouse is living in the U.S. after an illegal entry or a visa overstay. Possible answers to enter here include "adjustment applicant" (if you've started the immigration process for your spouse by filing a Form I-130) or simply "N/A" (not applicable). See “What happens if I reveal on Form N-400 that my undocumented spouse lives with me?” for details on this issue.
Divorces are a source of worry for some applicants. See “Can I apply for citizenship if I’ve divorced the person who got me my green card?” for details on that issue.

Why does USCIS need to know about your and your spouse’s previous marriages? One reason is to double check that you aren’t married to more than one person, which would make your current marriage invalid or be a sign that you’re intentionally engaging in polygamy, which USCIS considers to demonstrate bad moral character. Even if the double marriage was an accident, it can create a problem if you got your green card as a result of your latest marriage. After discovering this, USCIS could take steps to not only deny your citizenship, but remove your status as a green card holder. If you find yourself in such a situation, see an immigration lawyer.

Part 10. Information About Your Children
Most of this section is self-explanatory. As stated in the N-400 instructions, however, you must list every one of your children, regardless of whether they are:
• alive, missing, or deceased (in either of the latter two cases, you would enter "missing" or "deceased" in the first line of the address)
• born in other countries or in the United States
• younger or older than 18 years
• married or unmarried
• living with you or elsewhere
• stepsons or stepdaughters not legally adopted, or
• born out of wedlock.
If you neglect to mention a child on your citizenship application, and you then come back later with a petition to immigrate that child, USCIS may suspect that you are committing a fraud to help someone else’s child get a green card.

Part 11. Additional Information
These pertain to your eligibility for citizenship, in particular your moral character and the amount of time you have lived in the United States with a green card. If your answer to any of the questions is “yes” (with a few exceptions discussed below), you are at risk for your application being denied. See a lawyer.

Question 5, which asks whether you are legally incompetent or have ever been confined to a mental institution, does not represent an absolute bar to citizenship. It does raise the question, however, of whether you are able to understand and therefore take the oath of allegiance required for citizenship. If not, a disability waiver may help you overcome this hurdle. But definitely get help from an attorney.

Question 9 is particularly important, yet confusing to many applicants. There is no need to fear listing your membership in volunteer, charitable, or community organizations such as a social club, union, church group, P.T.A., volunteer corps, and so forth. These activities demonstrate your good moral character -- and you must prove good moral character in order to be approved for U.S. citizenship. But it’s a different story if you belong to a group that advocates world communism, violence, terrorism, or other perceived threats to the U.S., or if you were involved with Nazi activities in Germany. Even if you don't mention these in Question 9, various other questions on the form are designed to elicit this information. If you belong to a group that might fit any of these latter descriptions, consult with an immigration lawyer before going further. As for the dates of your membership, it's okay to write "APPROX" in front of the date if you can't remember it exactly.
If you became a permanent resident through a claim for asylum or refugee status, and you check "yes" to any of the questions in this section, you might want to consult an immigration attorney before going any further. Depending on the exact fact pattern, it's possible you could undermine your underlying claim to asylum or refugee status -- for example, if you answered Question 16 to say that you worked in a forced labor camp, you could (if this information wasn't already dealt with in your asylum claim) reveal that you are ineligible for asylum as a persecutor of others.

Notice that the many questions (22 - 28) about your criminal history are quite broad. In fact, you will need to mention any arrests by immigration authorities such as Immigration and Customs Enforcement (ICE) as well as any traffic tickets that you have received. You will also need to supply documentary evidence showing the outcome of your criminal case. That doesn’t mean that any or every crime will result in your being denied citizenship.
In fact, traffic stops or tickets ordinarily will not disqualify you from naturalizing. And USCIS does not (unlike with other crimes) require you to submit documentary evidence showing their outcome if you received only a traffic fines or if you were not arrested and the only penalty was a fine less than $500 and/or points on your driver’s license. (Also, don’t worry about listing parking tickets here -- just the incidents that happened while you were behind the wheel of a vehicle.) But you should consult an immigration attorney for anything more serious on your criminal record.

On Question 30F, be aware that even helping a family member to enter (or try to enter) the U.S. illegally must be accounted for with a "yes" answer.

on Question 31, remember that the "U.S. Government" includes its immigration authorities, so if you have provided false information on a green card, visa, or other such application, you need to answer "yes" here.,/span>

Questions 33 - 36 ask whether you have been in removal, exclusion. or deportation proceedings. If you have been recently summoned for immigration court proceedings, applying for naturalization will not protect you. Your citizenship application will most likely be denied, unless you are able to have the court proceedings put on hold until USCIS makes a decision on your naturalization application. See a lawyer for a full analysis of your situation; or if you answer “yes” to any of the other questions in this section about removal, exclusion, or deportation.

Question 46 is important; it asks about Selective Service Registration. If you are a male who was living in the U.S. or got your green card before or between the ages of 18 and 26, you were probably required to register for Selective Service, a list kept in preparation for a U.S. military draft. If you have not done so, you must either register with the Selective Service now (which you can do only if you are still younger than age 26) or attach a statement explaining why you did not comply with your duty to register.

In Question 47 - 53, you must demonstrate that you are loyal to the United States and will fight for it if necessary. If you are a conscientious objector, meaning that for religious or moral reasons you refuse to take up weapons or join in a war, you can answer “no” to Question 50, which asks whether you will bear arms for the United States. If your beliefs would prohibit you from providing any support to a war effort, answer “no” to

Question 51, which asks whether you would be willing to provide noncombatant services. You will have to attach proof of your conscientious objector status and request taking a modified Oath of Allegiance.) If you are part of a religion that prohibits taking any sort of oath (for example, the Quakers and the Jehovah’s Witnesses), answer “no” to Question 49, but submit a letter from your church or other religious body confirming your membership. (Include this when you send in your N-400.)

Part 12. Your Signature
If possible, sign your name in cursive, not printed letters. Cursive means a flowing style, usually slanted to the right, where the letters are connected.
If you are disabled and not able to sign your own name (for example, because your hands don’t work, or because of cognitive difficulties), you have two choices. You can either mark an “X” in place of your signature, or your legal guardian can sign for you. (After the signature, guardians should write, in parentheses “signed by [guardian’s name], designated representative.”)

Part 13. Signature and Contact Information of the Person Who Prepared This Form, If Other Than the Applicant
If a lawyer, paralegal, or other document preparer (including the designated representative of a disabled person) completes Form N-400 for you, that person must sign this section. If you completed Form N-400 on your own, or if you merely received some advice from a friend, neither of you needs to complete this section.

Part 14. Statement of Applicant Who Used an Interpreter
If, in Part 2, you requested an age- or disability-related waiver of the English requirement, this is where you and your interpreter must sign.

Part 15. Signature at Interview
You will not fill this out until your interview at a USCIS office.

Part 16. Renunciation of Foreign Titles
If you're allowed to go by a title such as "Princess," "Duke," and so forth, you will have to give this up in order to become a U.S. citizen. However, leave this blank for now -- you will not fill in nor sign this section until you are at your USCIS interview.

Part 17. Oath of Allegiance.

You must also leave this blank until your interview with a USCIS officer.