If you own a Mac built anytime in the last couple years, chances are you've got one of the most powerful music production tools available for a home recording user: Apple's GarageBand, bundled as part of their iLife suite.
In GarageBand, you can input music in three ways. One is pre-recorded loops. GarageBand is bundled with around 1,000 pre-recorded loops, with everything from guitars to percussion and brass. Second, you can input with any recording interface that's Mac compatible, from the built-in soundcard, USB microphones, or simple external interfaces. Third, you can use a MIDI keyboard to perform any one of the 50 included sampled and synth-based instruments. Expansion packs are available and very popular.
Let's take a look at how to create a simple song using GarageBand's included loops. I did this tutorial in GarageBand 3. If you're using an older version, you might find some of the menu choices slightly changed. Let's get started!