If you were not a citizen at birth the other way a person may gain US citizenship is through naturalization.  The first step to gain naturalization is to submit form N-400 if you are over 18 and form N-600 if you are under 18.  For naturalization of adopted children of US citizen parents use Form N-643.

For assistance with your case, click here to find Naturalization Attorneys near you or find an attorney in a different state.