How to Connect Real Instruments to Your Mac
If you’d rather not use the Apple GarageBand on-screen music keyboard to control software instruments, you can connect a real MIDI keyboard through a USB cable (on most newer gear) or a MIDI adapter (on older equipment). You can also connect other MIDI instruments, including guitars, woodwinds, and drums, and record onto a real instrument track in GarageBand.
To use your own instruments to record on a real instrument track, click the red record button when you're ready. Move the playhead to just before where you want to start jamming.
If the high-quality instrument is your own singing voice, connect a microphone (in lieu of the Mac’s built-in microphone) to the audio input port on the computer. Open System Preferences, click Sound, click Input, and then select Line In. Drag the Input volume slider to an appropriate level.
Good microphones are especially useful when you're recording podcasts.
MIDI is shorthand for Musical Instrument Digital Interface, a standard that has been around for years.