Playing music from your PC to an AMP.

Basically, I want to play all of my computer stuff (Mp3's, etc....there's some noise manipulation programs I have as well) out of my guitar amp. What cord do I need to do this?

An audio cable, duh. Your computer will have a 3.5mm (1/8") jack socket, your amp probably the larger 6.3mm (1/4") variety. If you can't find an audio lead with one of each variety at either end, just get one with a 3.5mm jack plug at both ends and a 3.5-6.3mm adapter (both very easy to find).

I doubt it'll sound very good, guitar amps aren't really designed as all-purpose speakers.  Plus line-level will be too high if the guitar amp is expecting the unpowered low level input of the guitar.  Might need to run your computers output level quite low.

YOU MUST SUCK YOUR OWN COCK AND SWALLOW THE DELICIOUS NECTAR
THAT WILL ERUPT FORTH

THEN FINGER BANG YOUR ANUS

There are sound cards that will do this easily, look around on amazon to get some ideas, they'll have a variety of sockets for cables, more sockets more expensive though.  It's like if you were going to play your computer through your stereo, but the amp output sound will only be in mono. 

Normal Guitar amps wouldnt be suited to it. If you have an acoustic amplifier however it should sound quite well... loud also  though.