You should have a great time! I'm just getting into group rides myself, and I have seen and learned so much on just a few rides so far. 18 miles is a great distance for your first one too.

This weekend, there was a crash in my group ride. It was because this one kid kept swerving all over the place, and his derailler grabbed some guy's front wheel and took the guy down. (no major injuries). After it was over, the kid who caused it had no idea that he caused it. None. Try not to be that kid. You can avoid it by driving your bike in a straight line. Try to keep straight and be predictable.

You'll probably be intimidated when you first arrive and see all these guys that are way more experienced than you, but then you'll start going, and you'll realize that everyone is really nice and willing to help you (as long as you don't know everything already). Each group ride I've been on so far has been a great experience, and I've made friends that asked me to ride with them at other times.

One problem that I'm still trying to work around is the fact that pacelines scare me a bit, so I'm just trying to keep straight and not kill anyone, but then I forget to drink my water.