go to www.usna.edu find out as much as possible, read through the pdf catalog found on the website. follow their suggestions in setting yourself up w/ the correct courses/classes to take.
get involved inside and outside of school in leadership type positions, if you are from a largely populated area you might have some of your HS graduates currently at USNA and try to talk with current mids to get an idea of life at USNA etc...
one more thing...since you are only a sophomore you can apply for STEM (science, technology, engineering, and math) program in the summer b/w your sophomore and junior year. then the following year you can apply for Summer Seminar. both of these things you can learn more about w/ the website i mentioned above
If you have not done so already, you may want to take the SAT and/or the ACT your sophomore year. I would not wait until your junior year to take it. Good luck and set your sights high.
Take the toughest math and science classes your school offers and excel. Really bust your butt in your math and English classes next year (11th grade) as these teachers will be writing mandatory recommendations.
Find a sport/activity/EC that you really enjoy and commit to it. Most say that's better than bouncing around at various things. And by commit to it, I mean really devote your energy to it. If you like acting, participate in every play even if that means working stage crew. If you like soccer, play on a club team in addition to your HS team. If you like running - do some 5K's besides your track team. Get the picture?