Getting hired requires you to be commissioned through one of three sources: ROTC, West Point or OCS. If you wanting to choose aviation, I would recommend either doing ROTC or West Point. If you choose OCS it is highly unlikely they will let you choose Aviation as a branch (I have only heard of this happening once and he had it in his contract prior to going to OCS). The branch choices are made prior to commissioning and is based on a number of factors to include your PT score, GPA, peer evaluations, commander evaluations, etc. Once you complete your Junior year for either ROTC or West Point you will rank all branches from your first choice to your last. If you want aviation this also requires you to pass an aviation flight physical prior to selecting.