To record yourself singing over your song or playing an acoustic instrument, you will need a microphone. If you don't have a built in microphone on your Mac, you will need to find an external one. Once you have the microphone set up, create a Real Instrument Track by going to 'New Track' under the Track Menu. Then click the track header of the Real Instrument track you want to record on, and click the Record button (the red dot). When you're finished, click the play button to stop recording.