I'd bring a change of underwear, socks, and normal toiletries like tooth brush, paste, shaving, deodorant, etc... If you're a female, pretty much the same things, but add anything feminine if applicable.
Either way, you can handle being in the same clothes from the day before for a couple of hours. No need to bring anything extra. You don't want to be carrying things if you don't need to. Nothing bigger than a back-pack like you had in high school at the most. And don't forget your boots if you already bought some, or you'll be buying another couple pairs.
If you've already contacted your sponsor family, you might even see if they'll hold onto your cell phone. This way it will be there for Doolie Day which is a day off you spend with your sponsor family in between BCT part 1 and part Deux. If you're an IC, your sponsor family is usually replaced by your coaches. At least that's how it was for my son and the football team. They all met for a breakfast and get together before inprocessing. The coach took all their cell phones so they wouldn't get locked up in BCT. Then, during doolie day, they went to the different coaches houses for the day. The coach handed them all their cell phones; completely charged; and said: Burgers, dog, drinks, etc... are in the back yard; have fun; and the guys called home for hours. Along with mingling and such. Of course they shared with those who didn't bring a cell phone.
I really like helping the process during the school year with new applicants. But these last 2-3 months when the discussion changes to "I" and "A" day, really is fun. ENJOY your last couple months of high school. Enjoy your friends and family. Complete your "Bucket List" of all the things you might have wanted from high school. Maybe it's apologizing to that kid you were mean to or got into a fight a long time ago and never resolved it. Maybe it's prom if you haven't had yours yet. Maybe it's talking and being nice to that kid that nobody ever talks to. Maybe it's earnestly thanking some of your teachers. Maybe it's having a real good chat with mom and dad / sisters and brothers and telling them how much you love them and thanking them for your support. This isn't the time to think that things are automatically understood or that people got over things. This is the complete end of a major chapter in your life. As a student, neighbor, friend, and most definitely as a son/daughter. You're embarking on one of the most exciting journeys you'll ever have. Some won't like the journey; most won't realize how great it is until it's almost over. But the point is, once you arrive at the academy, there's really no going back to the way it use to be.
Best of luck to all of you. Sorry for going long on a simple thread, but truly, this is one hell of an exciting time. Cherish it. later... mike....