Like a lot of answers on this forum, the answer is...it depends.
There are different paths, but taking college courses that parallel an SA freshman year is a good start. Whether at a university, community college, or prep school, you need to show that you can handle college level work.
What do you want to go to prep school for? Dedicated prep schools will help with academic drills and help you improve your SAT scores. Military Junior Colleges will give you a corps of cadets to be part of, with an atmosphere similar to an academy, plus a possible plan B for an associates degree. Staying at your local community college saves money, and you can often take the hard courses there. Of course, you can go to the state university and work on getting your BS/BA and apply to OCS for the service of your choice.
I suggest you contact the admissions officers from each of the academies you've applied to and get their recommendations. They may be able to make recommendations based on how competitive (or not) your application was. Then, contact the schools that you are interested after doing a bit of research on their websites. Remember, it's not how many students the school places, but whether YOU place into the academy.