I don't particularly agree with jmo about the universal importance of professional organizations, at least right away; that said, he's obviously found them useful, and many people do. So given that I also know that some people don't, I'd say, give them a shot for a semester or two, go to their meetings, get involved, and then evaluate if you think it is useful for you. This probably depends partially on what career you're going into.
The one thing that I would say that nobody has so far is that grades actually do matter, and especially early. Focus on working as hard as you need to to do good work in your classes, and build good relationships with teachers. It's going to be your teachers who people will want to hear evaluations from later on if you ever decide you want graduate school (or a lot of different academic and internship opportunities); and your grades will be crucial in all those things.
On that note, cultivate relationships with your professors. Don't just show up to class. Go to their office hours. I don't mean make up reasons to go; but if there's something you're struggling with, why not go ask about it? They actually like this, typically; and they'll definitely know better who you are and what you're about.
I said before that early grades are especially important. It's not actually the case that grades later in college are any more important; it's just that people usually figure it out by then, but by then it's often too late. Many times I have talked to people who started doing good work later in college, but who had already goofed off for two years and either not gotten into their major (probably not an issue in this case), or badly hurt their chances of doing something that they've realized they want to do (graduate school, a particular, hard-to-get job, etc.)
So do have fun, but as others said, remember what you're there for.