Here's a great NYTimes article that you should send to any college freshmen you know. Sage advice from grad students!
It got me thinking about advice I would have given had the Times asked me for some thoughts...
A goal for each semester was to have every professor or instructor know my name by the time I turned in my final. In large classes, this can be difficult, so you'll have to be creative. At the liberal arts college I went to my freshman year, it was fairly easy. I was the only person from Utah so naturally, people had questions. My biology professor would frequently ask me questions during lecture about plant life in Utah. Everyone else asked if I was a polygamist's wife. At the University of Utah, this was more difficult, so I would go to office hours or, probably my finest moment in undergrad, argue with the professor about his methodology. The blogosphere may like to know that the professor I argued with and I continue to email back and forth to this day. :-)
Get to know your TAs. Most of them are fabulous, though exhausted, people.
The mark of an educated mind is to be able to entertain a thought without accepting it. (Paraphrasing Aristotle.) Take classes that challenge your opinions and beliefs. I took an economics course where I found I disagreed with many of the students and the instructor. But, for hours a week, I sat in that lecture hall and played with the conservative thoughts. In the end, I kept most of my beliefs with the understanding that things are far more complicated than I thought they were at the beginning of the semester. I think that the purposes of education are to (1) teach you how to think and (2) teach you that you really don't know much at all.
Don't turn in a paper that you haven't proofread and don't turn in a paper without some sort of thesis statement. I can usually tell what sort of grade I'll be giving a paper by the second page. If you need help with editing/proofreading, most universities and colleges have writing centers. If you need help with a thesis statement or other paper specifics, well, that's why we have office hours.
Enjoy it! You get the opportunity to learn from
the experts. Don't be afraid to disagree and don't be afraid to change your mind.