Well, after you shake the can. . . . . . . . . .
There are some things to keep in mind, though. The first is to be patient. Don't try to get 100% coverage with your "first pass." If you spray too heavy on the first coat, you could wind up with runs/drips, or an "orange peel" effect. If you haven't worked with spray paint before, I'd recommend investing a dollar or so on one of those plastic "for sale" signs you can get at Wal Mart or hardware stores. Practice on that first. Vary the speed of your pass, and the distance you hold the can from the surface, and see what the results are.
Try to use "similar" paints. Enamels with enamels. Or acrylics with acrylics. Sometimes, depending on what specific colors you're using, this isn't possible, but definitely make sure your undercoat is dry before spraying the next color over the top. I've ruined several projects by just being impatient and not waiting for the paint to fully cure.
I'm sure some other folks will post with some tips too.
