Ah, the scare/shock wears off after a few weeks. Reach out to people in your classes (or anywhere else). High school is a great place to meet really interesting people.
Oh, also. Make friends with your teachers, if you can. It'll make things so much easier. If you need help with anything, you'll want to be comfortable with the teacher you're talking to.
Hmm...what else? Just...relax. I remember being terrified before I started (went from an all-girls private school with 120 students to a public high school with 1600), but it's been a blast. And while I kept my friends from earlier, my best friends now I met freshman year.

Sports, theater, that sort of thing, are also great opportunities to meet interesting people.

Oh, the nasty homework/all nighters people warn you about? Don't procrastinate too much; we never tell you that we've had a month to do whatever we're whining about. If you plan your time, you can often avoid it. Unless your teachers are psycho. But, really, coffee is your friend. If you do have to pull an all-nighter, I've found it actually earns you respect.

"When the people begin to reason, all is lost" - Voltaire

Eh, don't worry about high school. Make friends with people who are like you (through extracurriculars YOU like), talk to the people in your classes, learn to gauge your teachers' anger levels (depending on what kind of school you're in, they may either be very low or almost nonexistent!), don't break any of the major rules...hm, what else....Make friends with who you want and who you can be friends with--some of my best friends are 2 grades older than me and that doesn't mean I'm stuck up or think I'm better than everyone else, it just means I'm friends with the people I'm friends with and don't care what other people think. However, don't try to act like you know something you don't, and don't act like you're cool before you have friends. Just go with the flow and see who you meet. I walked into my new high school knowing exactly one other person who was going there, and worked my way up from there. However, if you're going into school with people you already know, make sure you don't ditch anyone for no good reason, but let your friends go if you realize you're not friends anymore. People do grow apart, and friendships change, and that can be good instead of bad.

and don't procrastinate a lot. :)

Oh and, if you're a girl, don't waste too much time in the morning trying to look good. Really. (if you're a boy or desperate for sex, you can stop reading now. :P) All those girls with gorgeous silky hair? Unless it's like that naturally, they're probably wasting upwards of an hour making it perfect every morning. And pretty hair does not get you a sense of humor, a brain, or really anything other than pretty hair.

ANYWAY, good luck! (and try not to pull an all-nighter if you don't have to. they're really not that fun or particularly helpful...except to those magical people who retain information without sleep. :P Having 4 hours of sleep is much better than having 0-1. Either way, coffee is indeed your friend.)

I'm assuming you're not going to boarding school, so I won't give you the dorm run-down as well. :)

And one last thing...DON'T FOLLOW MY INSTRUCTIONS UNLESS THEY MAKE SENSE!!!....and fit in the situation. :) being yourself is the most important thing, no matter how corny it sounds.
Whee, enough advice-giving for now!

i'm about to go through the same thing.

i'm starting high school in about 20 days. & all the advice that i just read on this page is basically what my friends, who are mainly 1-2 years above me in school, have been telling me for the past year. stuff like... joining clubs that you're into. and the procrastinating thing.

one day! *rocks back and forth* SHREIK!!! *continues rocking back and forth*
Bisexual by nature. Absolutely fabulous by choice.

High school? No big deal...it's kinda like elementary school with more people. Um...don't be one of the assholes. The assholes tend to be the popular ones, though, for some odd reason...

Stand up for yourself, don't let yourself get pushed around...

If someone doesn't like you tell 'emn f- off!