I am by no means an expert, but great that you are thinking about this early enough to get some things going. My son didnt get serious about it until early in his Junior year. (He is a senior now and the USCGA, USNA or USMMA is his goal). Some things I can think of:
-take the ACT at the end of you Sophomore year to get a baseline. Take it often to improve your scores. Concentrate on Math and English
-take AP, IB or Honors courses where ever and whenever they are offered
-get involved in leadership positions in clubs and athletics, and student government.
-play at least one varsity sport and try to become a team captain.
-get involved in community service through clubs or church, etc
-get or stay physically fit and within weight ranges.
-Apply for AIM, NASS or whatever Summer programs the academies offer in the later winter of your Junior year. (January/February)
-stay tuned to these forums....they are a wealth of information.
-have a back up plan and be ready to execute it should things not go according to plan. There are many ways to get a commission. Don't focus on just the academy way. Explore ROTC options and the ROTC scholarship programs
Best of luck to you!