Good to see you are considering USAFA. I hate to burst your bubble, but you aren't really early in considering a SA or in building your application. My DS in the class of 2020 started considering USAFA as a 7th grader and believe it or not started building his application in junior high.
USAFA will look at the total person so:
Academics
Challenge yourself and take a challenging schedule filled with STEM classes. Keep your unweighted GPA at 3.8 or above (to make sure you beat the recent median GPAs).
Take the ACT and/or SAT until you are well above the median average. The ACT has moved between 30 and 31 in recent years. My DS took the ACT each year from his 8th grade to 11th grade years. He improved each time and stopped after posting a 34 or 35.
Athletics
Be involved and letter in a varsity sport if possible. Earn leadership spots on a team if possible. Stay in shape and make sure to score well on the fitness exam. Practicing on the activities scored in the exam for more than a year should give you a leg up on most applicants. Try to max out in several categories.
Leadership
Boy Scouts, CAP or JrROTC are all good ways to earn leadership for the application. Summer Seminar and/or Boys/Girls State. The main thing is to lead and be prepared to show how your leadership made a difference.
Politics
Look for opportunities to meet your MOC and their staff so you are on their radar. Keep in touch with the ALO or prior ALO and ask for their advice on how you are tracking. They understand the process and can give you solid advice on areas to improve on or where there may be gaps in your application.
Use this forum and research other articles where you can learn more about the process and USAFA life. Best of luck.