Try and work out whether or not there'll be a goal. Or if your RP will be more character based. You know building relationships, going about their daily lives.  If you decide you want a goal think what would best work to get all the players on board, don't let them go to far off course. Make it known if you have a goal based RP that you are in charge and you want to keep everyone on task. But allow input if they think you're doing something wrong.  And consider that possibility.

A good goal could be trying to escape the Vault, and you can further complicate things some time down the road with the infamous Vault Tec experiments. Though some are more obvious at the start than others. Like Vault 69 Where one man is trapped with a thousand women, or some such number.