Select the track that you want to compress. Your tracks appear as long, horizontal strips on your GarageBand project window, stacked on top of one another. To select a track for editing, click the left column of the track. So, for example, if you want to compress the track containing your lead vocals for a richer vocal presence, click the vocal track in the left column of your screen.
Click the “I” button in the lower-right corner of your project window to open your "Track Info" pane. If you're using an old version of GarageBand, you will just see a simple “Compressor” slider (with numbers from 0 to 100) that you can move from left to right to decrease or increase the amount of compression on the track. Unfortunately, this limits your options for customization. If you use GarageBand '11, select the “Edit” tab and click your “Compressor.” A compressor pop-up menu will appear.
Adjust the threshold, ratio, attack and gain on your “Compressor” window. Your threshold reduces the volume when it reaches a specified level, i.e. a high threshold allows greater volume peaks than a low threshold. Your ratio determines the amount of volume reduction when peaks occur. Your attack refers to the speed at which the compressor reacts to volume peaks, and your gain can give added volume or presence to the track. If this is all new to you, experiment with different compressor settings until your track sounds just right. You can also use the drop-down menu above the compressor sliders to apply various factory presets.
Click the triangular “Play” button at the bottom of your project window to hear your changes. If you would like to isolate your track from the rest of the mix, click the “Solo” button in the left column of the track (indicated by a pair of headphones). This will allow you to hear only the track that you are compressing.
Click the red bubble in the top-left corner of your “Compressor” window when you finish making your adjustments.