See dear during ,the filling up of NDA FORM you will be asked your preference there u have to choose between army/air-force/naval academy after passing out the upsc examination for Nda,you will be called for your Ssb interview,after passing interview+medical u have to wait for the merit list if u are able to make a place in final merit list then u will be allotted the branch marked as the first preference in application form.But there is a term to get allotment as airforce cadre and that is: u have to score >=70 overall as well as in PCM.Best of luck !!If you are thinking to opt defence as your future perspective.May god bless u!