If it's an organized group ride - there will be a group ride leader. Talk to them. Tell them you are new. They will ask how much/how often/what speeds you ride. Then put you somewhere appropriate. If there is no group ride leader, talk to the people around you to get an idea of what the ride is like.

Other questions... Yes, they should split up into ability groups if the group is large enough. Someone (usually all) will hang around with you after you flat while you change your tube (don't bring a patch kit - bring a tube). If it is a "no drop" ride, if you fall off the back, they will wait for you, or someone will try & bridge you back to the group. If it is a "drop" ride - you'd better know where you are.

Drafting - if there is a paceline happening, try to take a pull at the front, even if it is just for 20-30 seconds. If you can't, just pull through & go to the back.

Bring enough water/gatorade/food to get you through the ride. Bring a tube, tire levers & something to inflate the tires with. Either know where you are going, get a map or stick with the group.

Have fun!