GarageBand '11: Before you start recording
You can record your voice, an acoustic instrument, or any other sound using your computer’s built-in microphone or a microphone connected to your computer. You record sound from a microphone in a Real Instrument track in the GarageBand timeline.
Before you start recording, do the following:
Connect a microphone to your computer and make sure it is turned on and working.
Make sure you have enough free disk space for recording. Each minute of stereo audio recorded at CD quality uses around 10 MB of disk space.
Select the Real Instrument track you want to record in, or add a Real Instrument track. Be sure to set the input source and monitoring for the track.