If you want to do this yourself, pull the wires out of the brake light switch. Is the light still on? if so, you have a short someplace. If not, the switch is activating the light and you need a new switch.

I assume that the pedal isn't resting on the switch and is activaing the light. You can check that by pulling up on the brake pedal and having someone tell you if the lights go out. If that is the problem, the switch needs to be adjusted or the brakes need to be fixed to raise the pedal.