You will need to have iLife installed on your Mac to follow this tutorial.
Select GarageBand from your Dock or Application folder to start the program.
Once GarageBand launches you will be presented with a splash screen. If you don't see the splash screen you can select New from the File Menu. Click the Magic GarageBand button.
You will be presented with nine preset song styles to choose from, each features a different selection of instruments and parts.
Move the switch to select whether you would like to play a looped snippet or the entire song while auditioning.
Click the Audition button to open the curtains and reveal the performers of your selected style.
Press the Play button to have the band start playing.
Click one of the instruments to bring up alternative instruments for that performer.
You can also click and hold an instrument to bring up alternative instruments for the performer.
Once you have found a combination of instruments you like, click the Create Project button.
GarageBand will now create a new project which contains all the parts and instruments you selected. You can now edit or build on this project as you would with a normal GarageBand Project!