For your first time in High School, it may seem like a maze. You're up to many new experiences, some good, some... maybe even bad.
Don't let people talk you into doing wrong things like doing drugs or smoking. Strive to be good, rather then bad. Friends help you get passed the bad moments, but know who to you really want to call your friend. People who stick by your side, and help you get through bad times. Those are the kinds of people you want to be friends with.
Also, try to 'balance' your life. Don't let sports (if you play them) get in the way of school or your social life, the same goes for spending too much time with others or with school. Keep it all balanced, try to fit it all in specific portions of your schedule.
That's really all I have, so all that I can say now is good luck, and hope you do well.