The one real advice that I can tell you is to stay focused....oh, and to manage your time wisely. You get a lot more free time at the U than you do at USMA so I would manage your time wisely. USMAPS is a great opportunity to fix yourself up before you get to the academy (academically, physically, and mentally). Take advantage of AI sessions and study groups too. As far as prepping for SAT/ACT, I would suggest taking advantage of the prep book they are gonna give you. You do test prep in class, but it's not enough to really prepare you for it. So if you really want to raise your scores try to find different resources that can help you.
One other advice I can give you is to have fun. Go out a few weekends with friends, join a club, and enjoy life before you enter USMA. That doesn't mean go do stupid things though, because you'll be representing not only USMAPS, but also USMA and most importantly the Army. So, as long as you have your stuff together, don't stress and have fun.
When it comes to nominations, fill out an application for one during the first semester. They will let you guys know when it is a good time to start the nom applications. I hope some of this helps!