Okay, so you want to get started on making music on the computer. It's probably a lot easier than you think. I'll teach you how to for free using a set of programs found over the internet. The following tutorial is only for windows computers. However, you can find other programs of the same general type for Linux/mac.
Tracker
First, you'll need a tracker. Psycle is an amazing tracker, and can be downloaded here. It is open source and has VST capabilities (don't worry if you don't know what either of those mean.) Unless you've had experience with a tracker before, you'll want to read the tutorial or else it'll seem too confusing. You'll need this or another tracker.
Examples of Other Free Trackers
Sound Editors
These are not necessary, but it's very handy, mostly for clipping audio files and exporting sound files as other formats.
Examples of Sound Editors
Music Theory
Don't be skeptical about this one, even if you think you know a lot about music. Learning more (or anything) about music theory and composition can help bring out what you want to create onto the computer. It adds so many more colors to your musical palette. Sometimes it can be frustrating to learn, but it's rewarding and gives a lot more life to your music.
Sampling
Sampling is taking audio from another source and adding it into your own piece of artwork. It can bring your music to new styles, and add a lot of depth.
Taking it further
This tutorial is only to start you into making computer music. While you are able to make decent material using these, practice and study will complement your music further. Pretty soon (or right away) you'll be impressed with the things you are able to forge out of these tools.
Cheers,
Nelson Doyle