Don't be afraid to ask questions. Don't study so hard that you don't have any fun, but don't have so much fun that you don't study hard. Go to class, because some teachers give credit just for showing up.
Don't jump into a relationship as soon as you get there, and if you do, don't expect it to last. Anyone that does you wrong, don't fool with them, but forgive them nonetheless. Don't do anyone else wrong either.
Don't go partying and drinking and smoking, etc, just for the sake of getting the experience, especially if you're underage. There are plenty of ways to have fun without risking getting into trouble.
I would personally rather have a few good friends than a lot of so-so ones. Popularity is no longer important once you've left high school. In fact, the "popular" ones are usually the ones that everybody runs out on at the end of the day because they are too fake or mean or try too hard to control their environment and the people in it. Be real.
Last but not least, if you are Christian, find a ministry to get involved in. Once you get out of the habit of going to church, it is HARD to get back in, so get involved as soon as you start.