I was just given an old airbrush gun/compressor that has been in storage for a couple years. Can anyone tell me how to get it good and clean so that it can actually be used on cakes?

What was it being used for previously?

I would probably keep the compressor if it works well and you can adjust it down to 10-15psi (I think that's the right level) but I would go out and buy a new airbrush. You don't know what's left behind in the airbrush itself and you won't know if it's truly clean IMHO.