Quick Guide:

First off all, you have to sand it using 400grit sand paper (wet or dry also can), sand paper can be found at local hardware shop or Ace hardware shop (3M Sand Paper is better). After sanded your PC case.... Wash your PC case and let it dry.

After that, apply Self etching primer (SEM brand is better if you need high quality paint job), or you can use Krylon Primer (Can be found at Ace Hardware), apply 3 or 4 coats layer to your pc case. Then leave it dry at least 3 or 4 days (normal temperature, do not put it on Sun dry). After that, you need to use tack cloth to remove dust and over spray. (tack cloth can be found at Ace Hardware also).

After clean up, use hot water to warm out your spray can at least 10min to 20min (Krylon Gloss paint is the best color for mirror gloss looks), and spray your panel case (30mins per one layer coat) at least 3 or 4.

After done painting, leave it dry at least 5 days.. if you wish to apply more like a mirror gloss, then you have to wet sandling it a few times, and use rubbing compound (I used Turtle Wax Rubbing Compound).