Good for you...that first organized ride can be intimidating, but you'll have a lot of fun. If you get hooked, there's a good list of West Coast rides up to 150 miles here: http://www.bbcnet.com/RideCalendar/RideListDate.asp
As for etiquette, it's mostly common sense. The fast riders will take off from the front and be out of your life in five minutes, which is fine with me--some of them tend to be what are known as ARPs, Arrogant Roadie Pr
!cks who get angry if you hold them up and tend to cut off and intimidate less experienced riders.
After that, you'll find your place in the group and just roll along. It's good manners to let people know when you're going to pass (just say, "I'm coming by on your left" or whatever), and to look back before you make any moves, so you don't run into somebody who's passing you.
Depending on your condition and the length of the ride, pace yourself to finish. It's really common to start fast, blaze through the first however-many miles feeling good, then wish for death in the final hour. Try to resist. Just go out at your normal pace (or a little slower, if it's a longer ride than you're used to) and if you have something left toward the end, then you can use it up at the back end, not the front.
Drink often, even when you're not thirsty. By the time you get dry, you're behind and it's hard to catch up. Eat judiciously if you're going to be out more than a couple of hours, but don't try anything new--not a different energy bar, sports drink, seat, shorts or anything else--on a long ride. Two hours into a four-hour tour is not the time to find out that your shoes don't fit and Gatorade gives you diarrhea.