It's good that you're thinking about this right now, because high school is the second-best time to start studying for the MCAT. (The best time, of course, is middle school, when your brain is less developed and more malleable—but better late than never, as they say!)
First of all, for MCAT purposes, gym and lunch are surprisingly high-yield. I would recommend eating the smiley-face fries and the sloppy joe sandwiches whenever they're served, and definitely try to beat 8 minutes on the mile run. Also, pay close attention during health class, especially the awkward parts that make the girls blush and giggle, because that stuff is going to be all over the MCAT. Take good notes during AP US history class, because I've heard that random trivia about Abraham Lincoln, Transcendentalism, and the Prohibition are all MCAT favorites.
That's all of the information that I've got for you at this point. Good luck with your MCAT prep!