Can't speak for AFROTC, but I'm currently a noncontract, nonscholarship freshman in AROTC. Had my heart set on CGA (or so I thought), changed my mind fall of senior year, and decided on Army ROTC far too late to apply for a 4 year scholarship.
It is very easy to become involved. I literally just walked into the ROTC building on administration day before the semester began, and they were very receptive and helped me get my paperwork going.
Once on campus and settled, go see the detachment commander or recruiting officer and talk with them. You will have to sign some forms and I think apply for a background check. You will probably also need permission from the detachment to enroll in the AFROTC "class" that meets once a week. When your paperwork is in order, start attending PT, and labs. I know that in AROTC once they know you are committed and involved (which you will demonstrate by attending as many PT sessions, labs, etc that you can), they will issue you equipment as needed.
If any of this is different in AFROTC, I apologize! Best of luck!