Don't do drugs, don't smoke, don't drink until you're the legal age, and don't be pressured into doing anything you're uncomfortable with.
Join a sport! Even if you're unsporty/unfit/whatever other excuse you make, it can be one of the best decisions of your life as they can lead to sport scholarships, friends, and a new passion!
High school is all about labels, unfortunately, and they're hard to break, so make sure you introduce yourself to be the best person you can be!