Create a new track in GarageBand by clicking “Track,” “New Track.”
Select “Real Instrument” from the window that appears, and click “Create.”
Place a microphone in front of your guitar amp.
Plug an XLR cable into the microphone, and then plug a 1/8-inch mini plug adapter to the other end of the XLR cable.
Plug the 1/8-inch mini plug adapter into the “Line In” jack of your computer.
Click the “Apple” icon in the top-left corner of your computer.
Click “System Preferences.”
Select “Sound” from the “System Preferences” menu.
Highlight the “Input” tab of the “Sound” menu.
Select “Line” as your input source, to use the signal from the microphone you plugged in.
Play the guitar through the amp, and adjust the sensitivity of the microphone by dragging the “Input Volume” slider left or right.
Click the “Record” button on the new track you created in GarageBand. This button will glow red when it is armed for recording.
Click the “Record” button at the bottom of GarageBand to begin recording the guitar amp.
Press the "Stop" button when finished recording.