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Before you set out to compose music in GarageBand, you should have a general idea of the instruments you'd like to use and the genre in which you'd like to operate. When you first load GarageBand, many of the available sounds aren't packaged with it. You can download these sounds for free by doing the following:  Click GarageBand in the top-left corner of the screen. Select Sound Library  Click Download All Available Sounds  Follow any on-screen prompts. MIDI instruments typically connect via a USB cable, so you may need a USB 3.0 to USB-C adapter for your Mac. If you do have a MIDI keyboard, you can skip ahead to the next part. Skip this step if you don't have a MIDI keyboard. Click the Window menu item, then click Show Musical Typing in the resulting drop-down menu. This will bring up a list of the keys which you can use to replicate piano keyboard keys. If necessary, you can change your Musical Typing preferences by doing the following:   Keyboard section — Click and drag the slider at the top of the window left or right to change which part of the keyboard is being used.  Pitch Bend — Press the + or - key listed in the upper-left side of the window to raise or lower this value.  Octave — Press the + or - key listed in the lower-left side of the window to raise or lower this value.  Velocity — Press the + or - key listed in the bottom-right corner of the window to raise or lower this value.
Determine the type of music you want to create. Download the GarageBand sound library. Attach a MIDI keyboard if needed. Open the Musical Typing window. Change your Musical Typing settings.