The academies look for 1. Academics (grades, course load, test scores, class rank, etc), 2. Leadership (clubs, scouts, CAP, boys/girls state, etc) , and 3. Athletics (sports, CFA).
(In that order of weight, if I recall correctly).
So, take the hardest classes offered at your school and do well in them.
When the time comes, take the standardized tests several times, and do well on them. (USAFA superscores)
Be active in extracurricular activities, and take some leadership in them.
Play sports. Earn leadership roles here, as well, if you can.
Stay physically fit (not all sports automatically cover this).