Listening while recording---dumb question?

I'm new to GB and kind of learning-as-I-go, but I can't seem to figure this out. This is my first recording, and I recorded an electric guitar track. I want to lay down a bass track but how do I listen to my guitar track while I'm playing bass? I'm seriously not a good enough musician to play bass to a silent track just looking at the waveform.

And on an unrelated note, even with Monitor On, I cannot hear my electric guitar through my computer speakers whole I'm playing (nor my headphones, for that matter).

Any help is appreciated!