Would also recommend you sit down with your college placement counselor and start working out your curriculum for your high school years so that you can be reasonably certain that you will get the courses you want and need. The SA's all award Bachelor of Science degrees, and that should be a big clue that they are going to load up on math, science, and engineering courses, probably for the first two years. In high school you should take algebra II, plane geometry, trigonometry, pre-calculus, calculus, chemistry with lab, physics (with lab if available), and do take the best grammar course(s) available. As noted above, it is important that you take on the most rigorous courses available, preferably AP.
Look for leadership opportunities in and out of school: eg, student government, team captain, Boys/Girl's State or Nation (check American Legion), Hugh O'Brien Youth Leadership (Kiwanis), Boy/Girl Scouts, CAP, JROTC, Junior Marines, Sea Cadets, so on. The SA's are looking for young people who can be trained to be leaders, so they are looking for young people who have some leadership training and experience.
BTW, start working on a fitness regimen that fits in with any sports in which you are participating, and be sure to include distance running and upper body strength work.
Start now to learn as much as you can about the service academies in which you have an interest; go to their websites and find out what visitation programs they have and when they will be available to you. For example, NAVY has STEM programs available to students entering grades 8,9,10, and 11 which are accepting apllications for this year's programs. The Naval Academy Summer Seminar, open to students entering 12th grade, will start accepting applications for this June's sessions after midnight tonight. If you are aiming to be a varsity athlete at NAVY you should visit <www.navysports.com> to find the schedule of sports camps that will be available.
Sometime in the next couple of years you should plan to visit the service academy(ies) that interest you. While the academies are quite similar, there are some significant differences in their programs, and it is important to you that you know what you are pursuing and whether it will be good fit for you.
Start taking the SAT and ACT exams this spring, but be certain you study for them. The academies super score the exams, meaning that they will give you the advantage of your best scores regardles of whether the score is from the ACT or SAT. Candidly, your math scores are more vital than your verbal scores, but get them all as high as you can. AIM HIGH! You are pursuing a high value, fully-paid scholarship to highly ranked schools - which looks better to you, a 26 or a 34?
Make contact with the local offices of your US Senators and your Congressman so you can get on the mailing list for any service academy events they may be hosting. You can get lots of information regarding their requirements for receiving a nomination. Your MOC's are the biggest source of nominations in the process; out of approximately 6,500 nominations awarded annually per DOD academy, about 5,000 will come from the Senators and Congressmen. You really need to apply for every nomination for which you are qualified; you never know which one will serve you best - and thai includes the Vice Presidential nom which is available to every US citizen AND IT DOES WORK! I have known two candidates who have gotten appointments via the VP nom.
There is a series of books available that address each of the academies with the generic title The ------ Academy Candidate Book. The books will provide you with lots of information regarding the subject academy as well as the service it serves; eg, Air Force, Army, Navy... etc. You may find them in the academy gift store or from Amazon.
I hope that gives you some ideas for your spare time for the next couple of years. It may seem awfully early to you, but there is a great bit for your to do and the time will pass much faster than you can imagine. Best wishes to you.