When you're making a GarageBand project, especially one with vocals (music or podcast), it can sometimes be useful for you to hear your own voice as you are singing or talking. The feature is called an audio "onitor" (not to be confused with a traditional computer monitor). There are two ways to do this.

To turn this on in GarageBand, simply show the properties for the vocal track you are recording to (clicking on the "i" button in the lower right-hand corner will open the properties pane if it isn't open already). Once you have done this, change the "Monitor" preset to "On". See the image below:

This usually works pretty good, but you may notice a slight delay (several milliseconds) in the response time. If this is enough to cause you anxiety, let's visit option #2.

M-Audio makes a great device called the iControl. This is a piece of hardware that connect to your Mac via USB. Not only does it allow you to use the monitor feature externally, but it gives you total control over your inputs. The price on this is $150 (significantly higher than free), but if you plan on doing a lot of recording, it's a great piece of equipment to have.