Well, first it's getting some animo. Then it's creating a world. Then there's plenty of time to join in. No rush.
It's not like you have to begin right at the start. Can always chip in somewhere along the line.
Futuristic: Set in the future, space ships, laser guns, tight leather outfits, mercenaries, pewpew
Medieval: Knights in shiny armor, jousting matches, battlefields, superstition and religion!
Steampunk: A magical world with a rise in steam based technology. Magick vs tech
Apocalyptic world: mutants, pollution, renegades, anarchy, everyone for himself. Huge guns!
The actual present world... ...