For the most part, take your cues from the rest of the group, because different clubs are different.

"Who rides where?" You can ride two abreast when traffic conditions permit, if you want to converse with someone, and if you feel your handling skills are up to it. The ride leader may or may not physically lead the ride, and there may or may not be a "sweep" rider designated as the very end of the pack. If it's not a paceline, then there may be times when you will pass or be passed by other riders in the group (mostly on hills) - alert them that you are passing by calling "On your left," and giving them plenty of room, same as you would do for a rider or walker who's not part of your group.

Google "vehicular cycling" for some good information on lane position - or search this board, I know there have been links posted to nice videos.

"How do you know when to stop/start?" You are a vehicle, so you're required to obey all stop signs and stoplights. Many riders will roll through a traffic control device after slowing WAY down and making sure there's no oncoming traffic (although this is still illegal, and if a LEO sees it, they're justified in ticketing you). Always signal when you brake, both by hand signal (same as the one you use in a car) and audibly ("Braking!" "Stopping!" "Slowing!").

Always point out obstructions in the road to the rider(s) behind you, by pointing at them with your finger. In some clubs it's customary to audibly identify the hazard, too, and it never hurts to do that ("Hole!" "Gravel!" "Roadkill!")

Do you know anything about your local club? Many clubs have specific rides for newbies, and groups that travel at varying paces. When you show up for your first ride, let the ride leader know that you don't have much experience riding in a group. Helmets are usually required for group rides, but you wear yours all the time anyway, right?

And have fun!