Know where your classes are before you go!!!!!!! (Unless your school is like my ******** school and doesn't give you your schedule until day 1...ignore my venting...just kinda annoyed) I second the homecoming thing. Go to all the events and football games, etc. Great way to hang with friends or meet new ones.
DON'T STRESS! Just because you're in a new school doesn't mean that it's a big deal, you still have your friends and the good ones will be there for ya too.
When it comes to clubs, don't do 5 thousand thinking it will look good. Colleges (I know, 'I'm just a freshman! It's still 4 years away!' but still, a good thing to be thinking about

) don't want to see a bunch of clubs that you came to 5 meetings of ever. Do thing you like (obviously) but things that you are good at. Example: If you're an English nerd, like I happen to be, find a newspaper, magazine, or yearbook. Do thing that actually apply.
Now that I have officially turned into a guidence office's pamphlet (and before I start saying 'get involved')...one last thing...PLEASE don't reinvent yourself. I hate the people that think they need to reinvent themselves for high school. Don't become emo or goth or super preppy just because you can! I beg you.
p.s. ~ Homework doing is a must...it's pretty simple how it works (well, it is here)...you don't do the homework, you fail.