Talk to your counselors. seriously.
Have fun, but not too much...and find something interesting, something that's different, something that changes from day to day. If you get stuck in a rut, or a rhythm at school, it'll suck. Sad to say, I had a period my freshman year where there was nothing interesting to do, none of my friends were fun, they just sat at their computers (I did too, but still), and I was probably depressed for about a month...I hardly recall anything in a 2-week period. Not cool.
Go to parties. like others have said, meet girls, pick up girls, take girls home.
Walk around town sometimes, look for something that you've never known was there. (Granted, not everybody is in a college town like Ann Arbor)
Something that provided me a lot of happiness: singing/a capella. It's very big in certain schools, east coast particularly. Plus, if you sing well, the whole girls thing comes into play again.
Early classes are up to you...as a freshman, you may not be able to avoid them, and hell, you might like them like one of my friends does. It is nice having the whole day ahead of you...but that whole waking up early thing....blech.
Just go, have fun. The second that you think you aren't, drop everything, find something new. College should have so much to give, that you'll never see it all. all you can do is try.
Oh yeah, don't fail your classes, that's bad.