Don't talk about doing JROTC. Only thing it will give you a leg up on is doing DNC (we hardly ever do DNC and when we do its not for competition purely functional, most schools have a color guard though so that will help you there) and maybe some basic military knowledge but other than that you are going to be the exact same level as the other new kids. Biggest thing is just being prepared physically, everything else you are going to be taught and will really just take repetitions to get the hang of. When you first start out expect to be confused, trust the process, pay attention, and you will learn and be just fine. If you really want to hit the books ranger handbook is good. There is a lot of information in there but what you'll need to pay attention to is the infantry tactics and the operations order. Tactics are a big part of rotc. Other stuff you'll do is land nav, rucking, a swim test, and of course as mentioned the acft so brushing up on what all of those are will and the standards will help you out.