GarageBand includes a metronome, which plays a steady beat (like a “click track”) to help you play and record in time. You can turn the metronome on or off when you are recording or any time your project is playing. The metronome always plays at the project tempo.
You can also set the metronome to play a one-measure “count-in” before recording starts.
Click the Metronome button 
in the control bar.
Click the Count-in button 
in the control bar to have the metronome play a one- or two-bar count-in before recording starts.
Choose Record > Count-in, then choose the number of bars from the submenu.