Depends on what kind of paint job you're planning, and what case you're using. A solid color paint job is pretty simple. The key is to make sure you tape over the holes on the inside of the case, also remove the motherboard backpanel if you can.
I completely stripped my case (all components out), carefully removed the buttons and LEDs from the front panel, removed all the fans, removed all screws and brackets (slot covers), then went over the metal with a piece of steel wool. Then wiped down the case with a cloth towel.
If you've never spray painted before I'd definitely get in some practice. Can spray paint is prone to spitting/dribbling so you need to know the right distance and the right method of spraying for best results. Since it's fast drying you can't really wipe up mistakes.
If you're planning on doing pin-striping that's a whole 'nother ballgame.
Decals are a good idea- just make sure you put them on before you start your clear coat layers.
I experimented with a few old cases so I knew how the metal would react. Again- cheap paint usually is latex based, and will peel very easily. If you're doing something like a two-tone paint job the tape will NOT come off the painted surface cleanly with latex paint. Go for acrylic- or better yet, go to a auto store and get High Heat paint.
Here's a good tutorial to get you started: