In Audacity, to record yourself playing an instrument, talking, singing, etc, you will press the Record Button (red dot). This will automatically start a new mono track. For the sake of this project, you will record your pre-prepared commercial script. You may record your audio sentence by sentence if you desire.

Once you have recorded your desired recording, press the Stop Button (black square).

To add an additional track, select Tracks > Add New > Mono Track. 

In my example, I recorded a disclaimer for my commercial on a separate track.