You must  to file the form N-600

click here to download the form N-600 with all the instruction

The fee for most applicants is $600. There are two exceptions:

The fee is $550 if filing for an adopted child.
There is no fee if filing for a member or veteran of the U.S. Armed Forces - proof of service required. However, the fee for a child of a veteran or member of the U.S. Armed Forces is the normal $600 fee ($550 if child is adopted).

click here for tips on filing the form N-600