Open GarageBand and select the "Acoustic Instrument" option. When the program opens, select the "Vocals" option on the right-hand side under the "Real Instruments" tab. Choose the type of vocals you would like to use. There are several from which to choose including "Live Performance," "Pop Vocals, "Female Basic" and "Male Basic."
Plug your piano into the MIDI input of your sound card. Make sure the piano is turned on and ready to play.
Press the red record button on the instrument track in "GarageBand." Begin playing the music you wish to use as the vocal part on your keyboard. The music will be recorded with GarageBand and will be heard as the vocal style you selected in Step 1.
Press the red record button again when you are done recording.