As a senior in high school, I would say to get good grades in whatever classes you do take, keep around a 4.0 GPA, and do whatever electives you want. Freshman and sophomore year I wouldn't worry TOO heavily about what lies 4 years down the road. It is good to keep your goals in the back of your head and make sure you don't do anything to screw them up. Keep it solid and then rock your junior year, that is when it starts getting serious.
I had no idea I wanted to pursue this until well into my junior year, so you're fortunate to have a head start on the game. Your mind may change in the next 2-3 years though, so keep that in mind. I would worry about getting out and being a kid at your age before you have to take on a lot of responsibilities.
To answer your question directly, if all that is on your mind is getting into USNA then bust your ass in AP classes and take on as many extra-curricular activities as you think you can handle. At your age though, you may not realize how quickly you can burn yourself out so be somewhat cautious in your selections.
Just my .02$, so take it with a grain of salt.