If you don't get accepted into the Academy on the first application, keep applying every year. It shows you really want to go and whatever you do during that first (or second or third) year in college will help boost your application scores if you do well at whatever it is you do.
Also, go for sports. Any sport. And run. A lot. You might *think* you're a good runner, but when you get a mile up in altitude you'll feel like you weigh 60 lbs more for the first couple of months. So run a lot, starting today.
Texas A&M produces a lot of military officers. I'd say that the ratio of academy grads to aggies in my squadron is about 3-1, not bad odds considering aggies are "merely" ROTC grads and don't get much (if any) extra consideration for pilot training beyond their records and instructor recommendations.
There are lots of ways to boost an academy application. Join clubs, and become part of the club's leadership. Having a pilots license helps a little and if you are accepted, it can help pave the way to becoming a cadet glider instructor if you want to do that as a cadet. As I said before, athletics helps even if you have to find a community team to join. Test scores... I think you can keep re-taking the SATs and ACTs every year, so keep taking them until you're not allowed to anymore. Advanced placement courses are good, not sure if getting an A in a regular class is better or worse than a B in an AP class though.
If you go to college, make sure you get all passing grades and get A's if at all possible. And join a freaking college sports team. Intercollegiate sports is a good thing. Become a class or club leader.
Regarding a major, I don't know what ROTC or OCS requires right now. The Academy couldn't care less what your major is, because all cadets take enough engineering classes that they *should* be given a minor in engineering, even the "basic sciences" guys who can't complete any of the specialized majors programs. You will need a good background in math or you'll get stuck in remedial courses. Almost everyone who goes to the Academy has had at least a year of calculus before showing up, so you'll be a bit behind if you have to start calculus from scratch. This isn't a deciding factor though, and they'll catch you up if you don't have any calculus when you show up.
And don't forget, you don't even get your application in the door if you don't get a nomination. That means you have to convince your congressional representative that you are 1 of the 10 people he submits that year from his/her district. There is usually an application process including an interview with congressional staffers, and the secretary (or administrative assistant) who handles the applications can be your best friend or your worst enemy, so convince that person FIRST that you are serious but not some jerkwad.