Do your homework (at least for the subjects you care about).
I know, I know, I sound like a nagging parent, but I'm dead serious here. I was smart. I understood everything I was taught, and reveled in my understanding.
Then I stopped doing homework, and after a couple of years I was lazy and stupid, putting in enough effort to pass and little else.
Do not, do NOT, let this happen to you. Have fun, make friends, enjoy the full 'high school' experience, AND come out smart on the other end. Not just for your future, but because when you're smart, you feel frickin' awesome.