More Basic Functions
If you enjoy singing, or talking, most Macs have a built-in microphone to where you can speak or sing right into your Mac and record it. You can make a radio voice Podcast or you can try a three-part harmony with yourself. Make sure you choose the "Internal Microphone" option in your Sound Preferences panel.
The "Line-In" is also important though.
Using "Line-In" will give you higher quality sound options with less background noise. You can plug in a microphone, keyboard or MIDI controller, or even a guitar (via a 1/8" line-in, USB interface line-in or even Firewire 400/800 line-in). In fact the Mac OS X includes several amplifier and filter effects to where you can plug right into it and use it as a recording studio or practice amp.
Of course any music theory knowledge and the ability to play some type of instrument will certainly add to this. Handily, even if you don't have access to a MIDI controller or guitar, you can still make sounds using just your typewriter keyboard. It's called musical typing. The other basic skill you will need is to be able to find and open your Sound controls in your Preferences panel. I've included some screenshots to show you what these basic functions look like. Note: both Keyboard and Musical Typing are under the Window menu.
There you have it. You should now have enough basic knowledge to get started recording tracks of music and voice to Garageband! Garageband will amaze you with its versatility and power once you become acquainted with its controls.
*special note: if you're plugging in a guitar (via 1/4" plug, USB interface, or what have you) you'll want to switch to MONO 2 for best results. It's a sort of quirk with the program, later editions make it easier to find this by clicking on the virtual amplifier though it is still somewhat hidden.