First, a couple general pieces of advice:
Push yourself to be excellent in all things. First, a service academy wants excellent cadets, and moreover they want cadets who aren't one-dimensional. They want you to be a top-notch student, athlete, and leader. While it's obviously not possible, do your best to be perfect at everything. This comes down to hard work. If you shoot for perfection, and you fail, you will end up with high standards for yourself, drive that far surpasses most of your peers, and a very impressive list of accomplishments. What to shoot for: Do get a 4.0 GPA, high ACT/SAT score, letter in multiple sports, score above average on each CFA event, and take leadership positions in your activities; if you've done all these, you put yourself in the position where you know you're going to be a top candidate. Knowing that you've done everything in your power to achieve your goals is reassuring and satisfying. Make the academies want you.
Don't make excuses. Some people I've met in person or who have posted on this forum feel that they can't do sports or other extracurricular activities because school is too demanding or they're not athletic. Likewise, I know athletes who don't try in school because sports are their only priority or academics don't come naturally. You can't do that if you want to go to a service academy. Each cadet takes heavy course loads with challenging and rigorous classes. And, they're expected to play a sport year-round. If you don't do it in high school, you don't prepare yourself for a cadet lifestyle, while also showing admissions officers that you weren't able to perform in several areas in high school.
Second, a few specific things:
Academic—60% of your Whole Candidate Score (WCS)
As often mentioned, take a standardized test early and often. A schedule that worked for me was to take it Sophomore year early spring, Junior year fall, and Junior year spring. Another tip less mentioned but more important IMO, take practice tests earlier and more often. I took a practice ACT sophomore year, and probably one or two in between each official ACT test. When you finish your practice tests, review every question you got wrong. Attempt to solve it again without looking at the answer, and then reason through why you got it wrong and what you'll do next time. I would write all of that down on a piece of paper, as according to every teacher, writing things down helps you retain information.
And, get As in every class. Don't give up in a class you're struggling with. Rather, redouble your efforts. And, it should go without saying, but in most classes, most of your points are on small things like homework, notes, etc. Do every assignment, well. This prepares you for tests and it raises your grades.
Leadership—30% WCS
Seek out leadership positions and make an impact with them. Consider activities early in your high school career that will help you get these positions. And, doing sports counts for this section of the application. Boy Scouts, athletics, and Boys State are all good options.
Candidate Fitness Assessment (CFA)—10% WCS
First, the CFA is pass/fail. Know that. If you fail one event, you fail the whole thing, and consequently, you're not physically qualified. Meaning, you can't attend an SA.
To pass, and excel, make physical fitness a part of your lifestyle. Lift weights regularly, have a pull up bar over a doorway or in a hallway. Do small amounts of pull ups relatively often. People struggle with them, but they're really not as difficult as you'd think. Do max effort sets once in a while, but it's really about doing 30-50% of your max several times a day. Lifting weights is important, as you build the muscle mass to make training for the other events a breeze (maybe not the mile). I guarantee, there is a direct correlation between CFA scores and lifting weights.
For running, the easiest way to improve is to join cross country, which I did for my junior year in preparation for the CFA. It's not necessary, but it's a certain to get you to a good mile time if you put in the work.
Overall, know your goals and know what you need to do. Be familiar with the process and get WCS points. Have high standards and adhere to them.