If you all are assigned individual lockers, don't offer to share with a friend (no matter how unfortunate a location theirs is in). It can cause SO many problems...
Become friendly with your teachers. You may not want to go all teacher's pet (not that there's anything wrong with it...) but having them on your side will help enormously.
Work on projects/papers and study for tests in advance so that you are not waiting until the last minute to do everything. (And if you succeed in doing this, let me know your trick...)
Buy your own calculator for math.
If you ever get the opportunity to pick a seat, know that teachers pay a lot of attention to where students choose. The back means that you're there to goof off with friends. The front center means you are a bit of a geek. I've found that the safest place is along the side about 2-3 rows back.
Take some academic electives to get yourself thinking about what you want to do after high school, but allow your self a complete blow-off class once in awhile.
Don't show up on the first day of school without paper and pencil. You will be forced to borrow them from someone and forever be known as the person who wasn't even prepared for class on the first day of school.
Eat lunch. Many people in high school opt to just gab all through lunch and not actually eat anything (or they just eat french fries and call it a lunch). Eating a relatively balanced lunch is good for your body and good for your mind.
Get involved in extracurriculars, either at school or in your church or community. It's a great way to make some friends outside of just your classes and make some great memories.
Smile a lot. Be friendly to everyone and treat everyone you come in contact with with respect. It can be pretty difficult, but when that respect is reciprocated, you will feel great about yourself. Don't be afraid to chat with someone you don't really know all that well, you don't have to become best friends, but it is nice to have someone in your classes you are friendly with...you never know when you'll have a group project or need someone to borrow notes from.
Just because everyone else is doing something doesn't mean that you should. Do you really want to go through your life being exactly like everyone else?
Be yourself. Relax. Have fun. Study hard. Let me tell you, those four years, blink and you miss them.
(I had fun with this post...can you tell?)
"Never doubt that a small group of committed people can change the world. Indeed it is the only thing that ever has." --Margaret Mead