I highly recommend extra-curricular activities. Personally, my first 2 years of high school I stuck with chess, and I was part of the varsity team. While it wasn't all I had, it at least made school bearable, especially because I was too lazy to do well at my actual classes.
I also recommend finding a group of friends that you be around all the time. While you may not keep those friends during later years, they, like extra-curricular activities, will make life bearable. Friends are also a nice perk to have when they are all in your classes, making studying a blasty-blast.
But, most importantly, don't let any bad things take over your life, such as drugs or drinking. While you will probably try and even begin to use them during your high school career, don't let them control you, because I've seen people completely ruin their lives with the stuff.
Aside from that, just have fun. While I think that ProductofChina's advice was good, if you focus on just that, you won't have a good experience. High school makes you who you are, and it will easily be one of the best times of your life. Hell, I've been out of it for 3 months and I'm already lamenting the mistakes I've made. However, I'm also cherishing the memories I have of it, and I always will. So will you, once you look back on it all.
Just don't F&#$ up...