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You need to clean and sand (quickly, 400 grit) the insides of your case first. Remove rivets (if you want), cover parts like knobs, plastic pieces etc. with tape.
Spray primer (if you want, depends on whether paint sticks to the metal). Test the paint on a hidden side. If it sticks well, you really don't need primer. If it doesn't stick will, you need a coat or more of primer.
Spray the paint, let it cure (see instructions on can for duration) and you're done.
On paints for internals, flat black or satin black.
if you already had a pc inside the case, then you need to clean it for sure, there will be dust over every thing and it will show up after you lay on the primer/black, sanding may not be needed if the inside of the case has any sort of texture. if it is smooth, then you need to sand ( i feel 400 is to rought for bare metal, and will require atleast 2 coates of primer, i would say go with 800/1000 grit and a single coat of primer, flat grey)
more indepth
the next step depends on how anal you are about your finish. after you sand, get the dust out (canned air/compressor) and head straight for the primer, or... sand, blow it out, and hit it with a piece of cheese cloth, (do not touch any surface you have touched with the cloth) primer, 1200/1500grit paper, primer, 1500/2000 grit, paint... if you go with the longer procedure, you will end up with a flawless automotive quality finish.
( i went so far as to hit my case up with a hose, paint thinner, and then cheese cloth because there was some sort of an oily substances on the mobo tray... (probably oil used for lubracating the bit used to tap the mount holes) )
keep in mind, any thing you dont want paint on, needs to be covered/blocked off, this means that little crack at the bottom of the case that dosent look like much, the seconed the paint hits it you will discover its on your exterior finish as well.
and i vote for satin black... its got a slight reflective quality to it and wont show prints like true flat black/black primer will