I would recommend running through the
audio recording tutorial.
Assuming you have a good audio input signal, for the first track you simply:
• Go to the tracks window
• Select an audio input and output for a track
• Record-Enable that track
• Press record and play/sing
Then repeat on another track for the vocals. Note that when you select the output you probably want to select a different output channel so that you can adjust faders in the mixer window independently.
"Overdubbing" is not exactly the right term although I understand what you mean. I think all you really want is to record 2 different audio tracks.