how do i add distortion in audacity or garage band?

There's a few ways to add distortion in GarageBand. Probably the easiest is to open an Electric Guitar track, and select an amp setup from the drop-down menu near the top of the Track Info pane. From there you can adjust the levels on the stomp boxes (just start clicking on stuff and you'll figure it out yourself quickly), and you can adjust levels on your amp by clicking on it as well - you can even switch to different amps while keeping the same stomps (using the left and right arrows), as well as mix and match the actual stomps themselves by dragging them in and out of place.