Later down the line, Roll20 will have a marketplace set up. From there, one will have the ability to purchase pre-made campaigns/modules and/or art assets like dungeon tiles, maps, tokens and etc. This being the beta, you'll unfortunately have to put something together yourself until we have a marketplace established where you can import someone's campaign. So long as your players don't mind a pretty basic setup, you can wing a lot of it in roll20. The d20 systems don't need much setup in roll20 since there isn't much in the way to play a game beyond a battle map, tokens, and dice. If you don't already have a campaign that you're currently playing, a lot of the big games sell little module books (in pdf form too!) that you can play. You can take the battle maps from that module book and plop them into Roll20, you can take the drawing tool and scribble the map out that way or you can just find some tiles in the art library and build the map up that way. Smash up some tokens using the art library in the program and that's honestly all you need to get started if you and your players don't mind sticking with paper character sheets and paper GM notes.