Go for it! And, no don't wait.. the harder your course load gets, the more excuses you'll make.
Some of my best friends today are guys I never would have met (or probably even liked, for that matter) without having shared the whole fraternity experience. And, I'm talking about the "boring" stuff when I say frat experience. Working on projects, figuring out creative ways to get 60+ guys to do something they didn't want to do, volunteering, etc., etc.
It's just like anything else, it's what you make of it, and what your trying to get out of it. I saw guys who were really good at all the social stuff, but never took a thing from the overall bigger experience and just kind of faded out. I also saw people who came in without all that (very shy, didn't know how to play games or whatever). They wanted to grow as a person, and gain experience being apart of a team, build leadership skills, and what not, they could and really did thrive, and became just amazing leaders later on.
To answer your questions though: Don't worry about it, and just be yourself! My fraternity would invite people over to hang out a few times or go watch some game or something before ever even talking about parties. This was a way to weed out people only interested in that. Not everyone just gets a bid to keep the organization alive like a few posters mentioned.
For your situation, (which isn't unusual, and probably everyone else is also worried about it) this would be an awesome time for you to "get in" and get to know guys in a non-party setting. Get to know people like that, and the social part will just fall into place. You'll be hanging out with new friends rather than being worried about being in a big crowd or drinking or whatever.
You got a lot of advice, so I won't go on. I do have things I didn't really like as well but overall it was a really great experience. E-mail me if you want more, there's a pretty good movie I can recommend that is the most "right on" portrayal of Greek life, and the lead character is some guy without any social skills who is concerned about the same things you are. (If you watch though, don't ever tell your brothers because they will make fun of you. However, always, always, be trying to figure out a way to bring it up in conversations with girls..)