This free download from the App Store works well with MIDI instruments, but if you don't have a MIDI keyboard, you can turn your Mac keyboard into a virtual musical instrument.
GarageBand na-arụ ọrụ nke ọma na ngwaọrụ MIDI, ma ọ bụrụ na ịnweghị keyboard MIDI , ị nwere ike ịgbanwuo keyboard Mac gị n'ime ngwá egwú egwu.