Download a free music production software program. Check the resources section below for a direct link to a list of programs. Most of the programs offer a wide range of sounds that can then be crafted into specific notes and pitches. Once you create the harmony, melody, and rhythm -- the song is finished unless you want to add vocals.
Download Audacity. Audacity is another free audio program that will allow you to record vocals on the song if you wish. You need a computer microphone as well, but most PC's are shipped with one built-in. Recording the vocals on Audacity is very easy. Make sure the microphone is working, create a new track, click on the record button, record, then click the record button again to finish the process.
Export the file. You can transform the song into an mp3 format that will work in your mp3 player. You can save the file into Windows Media Player and listen to it there. You can upload the file to your MySpace profile and share it with the world. Whatever you decide to do, it's completely up to you!