First of all, select a scenario with the autocoach duh and you will be greeted by a few levers.
Once your passengers have boarded, press B to ring the bell once. This should trigger the fireman to push the reverser into forwards. You can then release the brake by pressing B three times. You will then have to press both E and A to set the regulator and you're good to go!

The autocoach is a right pain to drive. For instance, half the time, I can't ring the bell. Other times I ring it and it rings twice automatically. Do keep this in mind before you RAGE QUIT!