Although I'm not a pilot, I too was in your shoes chasing a dream. The board has many experienced military aviators that will give a great advice. Besides making good grades, being active in sports, and showing leadership skills (running for student government, become president of club, boy scouts, etc), I just want you to be aware that you need to keep up with the current events of the military.
Case and point. When I was in ninth grade military pilots were plentiful in the 80's. When I enlisted in the Air Force and was stationed at a UPT base, I spoke with the pilots going through flight training asking them their education background. I would get a variety of majors ranging from History and Literature to Mathematics and Sciences. When I finally seperated from the service and went to college the Air Force was going through a reduction in force and wasn't accepting officers without a hard science degree (Math, Physics, Engineering, Computer Science, etc)
If you really want this, my advise is to stay informed on the changes in the what military is going through. Keep in mind you have at least eight years before you will be entering service. If you goto college out of high school then you can qualify for a pilot slot by your Junior year in college. Stay focused and good luck.