Congratulations on your early decision to pursue an appointment to a service academy; it gives you a valuable ally - time. Talk with your guidance counselor about your decision and start lining up the courses you should be taking to prepare you for the rigorous academic schedule ;you will be encountering. Ensure that you take the courses recommended by the academies, all of which basically are engineering schools awarding their graduates with Bachelor of Science degrees - even if you major in English, political science, or economics. You should take your courses at the highest level available to you be it honors, AP, college equivalent, etc. Be sure you fit in Chemistry with Lab, Physics, Algebra II, Calculus, and Grammar/Composition.
Participate in sports, not just as a face in the crowd, but as a leader. Also get into other clubs and organizations that interest you and go for the leadership positions, but don't overdo it or get into gaming the admissions process. Also be aware that these are academic scholarships and don't sink your grades because you are over-engaged in ECA's.
Start participating in events that will give you additional insights into the service academies in general and those that interest you. You can apply to more than one SA as well as the ROTC programs. Be prepared and be prepared to be flexible, but don't waste this valuable ally of time. Use it wisely and best wishes.