There are a few things that I wish I knew freshman year or had done differently, and I'd like to enumerate them. Actually some of them I did and found amazingly helpful.
1. Find something to escape in. Find something that makes you completely forget about the assignments, upcoming tests, etc. Life gets so much better when you have a few hours a week when you're completely focued on something that isn't school.
2. Get off campus earlier. I didn't do late night freshman year, I didn't go to the Waterfront, I didn't get to Shadyside more than twice.
3. Try things that you might not like. You might find your new favorite thing.
4. Don't ever hate something. Think that it might be different the next time. I've met people who had one bad experience with a food genre and they think they hate it, but they just had something that wasn't great.
5. Go to concerts of bands you only kind of know, or your friends like. They might show you new music you love.
6. Run an organization. Or at least be a really active officer. You get to see how people in charge you thought were awful were pretty awesome, and you start realizing how much people in charge who seem awesome are gods. Plus, every interview I've had has talked about me running CIA.
7. Date more, when you know it won't work, end it. There are tons of opportunities to date, why try and patch up a bad relationship?
8. Pay attention in class. The things you think you can ignore because they won't be on the test? They'll be in an upper level class, and you'll be at a loss because everyone else understands them.
9. Take a writing course every year. I have noticed my writing skills seriously slipping as I haven't written a paper since sophomore year.
10. Do things that are more practical than theoretical sometimes. For example, learn how to install MySQL and Tomcat and a few libraries if you're a CS major. It's not really covered in classes, but it will help you get jobs - people know you know how to get dirty and you understand the application, not just the theory.
11. Start research in your sophomore year if you can. It's lots of fun, lower stress, and very rewarding.
12. Tutor or TA. Helping people learn teaches you how people learn. You will become a better teacher or explainer (and therefore, a better friend and neighbor).
13. Live in a freshman dorm. I'm still really close with people from my freshman dorm.
14. Join a club. This is related to numbers 1 and 6, but it gives you another base of friends.
15. Take advantage of culture on and off campus. I've been to a bunch of shows downtown, and on campus - and it's made life a lot more fun.
16. Be on a first name basis with your advisor. They can be your biggest advocate in classes, jobs, and life.