I agree you need to check with the det regarding their process. Overall, typically it is when you go to orientation you just tell your academic advisor/counselor when you sign up for classes that you are going to be joining AFROTC.
My best advice is that you need to keep re-taking the SAT and ACT this spring. Yes, it will not impact you regarding college acceptance, but if you try for a scholarship next yr a higher BEST sitting will be to your advantage. The avg HSSP recipient has a best sitting SAT close to 1300. Also realize that your intended major will be a player in the equation...tech or non-tech.