There was a recent thread similar to yours to which I responded in substantial detail... it was a couple of weeks ago, so it probably will be someplace in the first 5 or 6 pages. Also you could try a search.
Anyway, some important things for you to be doing with your headstart are:
Meet with your college counselor and start laying out your courses for the next two years to ensure that you are going to get the courses you need; eg, plane geometry, pre-calculus, calculus, trigonometry, chemistry with lab, physics (with lab), grammar and composition. Go for the highest level courses available, preferably AP if available. Take the AP exams as they can earn you course validations at the SA's.
Contact your Members of Congress' local offices and get on their list for the service academies. You are going to have to get nomination(s) in order to be eligible for an appointment. The MOC's also sponsor information meetings for prospects at which they explain their process for selecting nominees for the academies, and there frequently are representatives from the academies present to discuss their academy and respond to your questions and concerns.
Visit the websites of the academies in which you have an interest, and get familiar with their admissions process, what visitation opportunities they have, their majors, etc. USNA sponsors STEM programs for your grade level for which applications are being accepted at this time. After your junior year there will be the Naval Academy Summer Seminar available to you. (Check <www.usna.edu/Admissions>) There also are sports camps you might consider (Check <www.navysports.com>).
Consider all of the service academies as well as the ROTC programs. Select the ones that seem to meet your expectations and objectives - they all present excellent opportunities for education and careers. While there are many similarities among the SA's there also are some important differences; it is important that you learn about both.
Do take the SAT/ACT exams seriously, because the academies do! Do not get suckered by the median scores on the Class Profiles; they tend to be lower than desired or expected by the schools. As you tour the serviceacademyforums site the next few weeks, take note of how many times you see someone bemoan the fact that their scores were not higher. It is to your advantage to have your SAT's over 700 and your ACT's in the 30's - you will rest better, trust me.
All of the academies are looking for young people who have leadership experience, and have a record of achieving as leaders. Seek these opportunities, but don't overload. Typically about 60% is ascribed to your academics (class rank, gpa, SAT/ACT, etc) so ensure that you don't short- change your studies.
Best wishes to you, and I hope you found some value in the above.