Are you asking about creating a song on your iMac or iPad/iPhone? This is the GarageBand for IOS devices forum
On your iPad/iPod you can set the key of your song from the Control bar in the "My Song" view. Click the wrench-icon: Song Settings button
Then select "key" and set the key to Ab minor.
On your iMac select the correct key, when you create the project, or change it afterwards from the LED display. Click the leftmost icon in the LED display to change to "Project" display, then click the key and set the key to Ab minor.
GarageBand will display this as G# minor, but the results are the same.