Then you can join as an adult member of staff. Your first role will generally be as a civilian instructor (CI), where your role will be to teach various subjects in the ATC syllabus - don't worry, all the training is provided! You may choose to stay as a CI, or you can decide to go into a uniformed role - either as a Non-Commissioned Officer (NCO), or as a Commissioned Officer. As an NCO your role will still primarily be instruction, but you will have more responsibility and you'll also be involved in the drill and discipline side of the corps. As a Commissioned Officer, you'll still be involved in teaching, but you'll also become more involved in the running of the squadron, and after a few years you may well end up running a squadron of your own!

Note: no previous military or cadet experience is required to become adult staff - although it is helpful of course - just a level head, a sense of responsibility and a willing to learn, and to teach!