From Apple's site
"GarageBand 3 features a built-in instrument tuner to help you get your instruments in perfect tune. To use it, enable a Real Instrument track for recording (click the red Record Enable button on the track). In the time display, click the tuning fork icon in the upper left corner to display the instrument tuner. Make sure that your instrument is connected to your computer, and play a note. The tuning meter will tell you what note is being played and how far off you are from the proper pitch-shoot for the zero (0) mark. The tuner displays a green light once you've hit perfect pitch."