1) First things first. Prioritize your tasks then set reasonable goals. 1st task is getting through Basic Training. You accomplish this by being a good squadron mate. Continue to run, run, & run. Continue building up your core/upper body. You don't want to be that Basic FEMALE who makes everyone repeat exercises considered by some to be unpleasant, esp. when performed at 7,000 ft. The guys will think 'typical' and you already know what the females in your squadron will think/say. After Basic, the next task is passing 1st Semester. This semester knocks a lot of students out of civilian colleges; students who have a much smaller workload then you will have. Sleep & time management will take on whole new meanings. I guarantee you sleep will become your 1st favorite activity & will remain so for all 4 years. "The days are long but the weeks are short"...or so they say. You absolutely cannot fall behind in academics. Being on Academic Probation (or Physical or Discipline) isn't as much fun as it sounds. That being said, the Academy will introduce Club participation at the appropriate time so don't worry, you will not miss anything. Being in shape physically & academically will allow you to enjoy club activities as a stress reducer instead of another time suck which will unnecessarily increase stress.
1a) Enjoy the rest of your Senior year. You will only have one. It's the best time to hang out with your HS friends before you all split up. Have fun but don't do anything crazy (Have to say it, I'm a parent, motorcycle riding, bungee jumping, DUI, basketball, [leading cause of ACL's & concussions for girls], etc.) that would jeopardize your appointment. See if the whole family can getaway for a family weekend before you go. Those memories will come in handy later, for you and your parents. After all, they are sending a girl out to Colorado & will be getting back a woman in very short order.
2) Lots of older threads address this issue if you use the forum's Search function. Agree with 16lpontzer that it's a broad question. Congratulations and good luck!