You are probably still too young, but CGA has a summer program called AIM for rising seniors (so you apply the spring of your junior year). It is a week long (there are 3 separate in person week long sessions) and it gives you a good taste of what swab summer would be like. Depending on your location, you could also visit CGA to see the campus and learn more about the Academy itself. In addition to reading over everything on the uscga.edu webpage, find out as much as you can about the Coast Guard mission in addition to just the Coast Guard Academy to be sure it is something you want to do. US Coast Guard Academy has a Facebook page and an Instagram page where they periodically post information. On Facebook, Paul Duddy is a photographer "pduddy pics" who is on campus almost every day so you can see a bit of what cadet life is like through his photos.
If you already know the Coast Guard Academy is for you, your main focus is academics, athletics, and leadership. Service is important as well. Make sure you are taking the highest level STEM classes that you can and that you are doing well. Make sure you are showing leadership which could be through a typical leadership position but could also be through service or other areas. And make sure you are participating in sports/athletics. You can look up the PFE requirements online, but you also just want to be in good physical shape overall. It still may be early, but it never hurts to reach out to the Admissions Officer for your region.