So I'm still pretty new to being a GM. If my PC say, look through a gas station, what's the best way I could decide what they get?

Catachan23 gave us some ideas:

http://community.fantasyflightgames.com/index.php?/topic/133579-scavenging/

I usually ask for an awareness roll (or the equivalent in whichever system) and then make up something based on how well they rolled. If they roll badly, they find nothing useful; if they roll average they find basic supplies and such; if they roll well, they find something unexpected and cool. In the gas station example, that'd be a shotgun behind the desk or some such.

I try to keep it down to one roll per room, just so things don't get tedious.

If there was actually something I wanted the PCs to have that could reasonably have been found there, I give that to them too (regardless of roll.)