Academics. Athletics. Leadership. Excel in the most difficult classes offered. Take both the SAT and ACT, early and often. 1500/35 is the min goal. Varsity letters, team captain. It isn't too late for Scouts, it is a 2-year trip to Eagle, or Venture Crew. Boys/Girls State after your junior year. Lifeguarding is a great job that has a leadership component. Junior Volunteer Fireman provides great training, stories, and examples of leadership. You want to make yourself so valuable the academy can't go on without you.
Go to the USAFA website and study it thoroughly, you will find the recommended academic profile. You can look at previous class profiles. The level of competition is fierce.
You have time but don't delay. Good luck!