Create a new music project in GarageBand.
Name your song in the New Project pop-up box. Select where you want to save the project.
Move the tempo slider in the New Project pop-up box to set the desired tempo. Select the appropriate time signature for the song, and designate the key if desired. Then click "Create."
Select "Control" in the GarageBand menu once you have created the new project. To change the tempo, go to "Control" and select "Show Tempo in LCD." Click on the tempo on the LCD display to change.
Turn on the metronome by selecting "Control" from the GarageBand menu and then clicking on "Metronome," or press "CTRL-U" on the keyboard to turn on the metronome.