I use Garageband now, but I used to use Modplug Tracker, which is free...
http://www.modplug.com/
The best way to learn with Modplug is to load other people's music (preferably .IT format) and see how they did stuff. Just look around for Impulse Tracker (IT) music/mods/tracks.
It's not that easy but then none of them really are.