If you're going to go to college, get a 4.0 gpa to make it easier on yourself when applying for scholarships. It doesn't take too much effort. Or take as many AP classes as you can and just do your best.
Join a club or two. You'll meet people and have fun. Maybe play a sport, if you're into that. It also helps when padding your shit up for university applications.
Also, even though I recommend getting straight A's and joining clubs and all that bullshit, just relax about high school. Be laid-back about the whole thing, the world isn't going to end if you make some mistakes.
Get to know your teachers on a personal level. You might need that connection later on.