If you're just starting a thing, you probably need to prep some starting situation, whatever "starting situation" means for what you're expecting/intending to run. For a sandbox, you'd need some of the local area and what's around it--things for the PCs to find out about and do; if you're running a game that's more about a narrative (not necessarily a pre-written one--writing up the whole expected narrative seems very much like the opposite of no/low-prep) you'll probably need a starting situation--set up some things, add the PCs, throw stuff at a convenient fan. If you're not pre-planning for specific PC actions, though, you really don't need to prep much, if you're willing/able to respond to the PCs and improvise; obviously, that's not a forte for all GMs.