Well, first you need a tune to go with the lyrics, it helps to read the lyrics and hum along (or sing along) to them in your mind if you don't already have a tune to it. The next step is usually to try and record vocals or instrument(s) for it. Do you sing or play an instrument?
Ok, cool. There are programs out there you can use to synthesize guitar/bass/drums/any other instrument you want. I recommend Cubase, it's what I use. I use Beatcraft, a drum sequencer, for the drum track. You can pretty much get them anywhere. The one thing you will need is a microphone, and probably a makeshift popper stopper to help the screaming come out right