YOu cannot just apply for US citizenship.  You first have to be a permanent resident (green card holder) and maintain that status for the requisite period of time (3 or 5 years, depending on how you get that status) before you can apply for US citizenship.  In order to become a US permanent resident, you have to be petitioned for that status, either by a qualifying family relationship (parent, child, spouse, sibling) or a US employer on the basis of a job offer.