Check the brake-pedal switch. This small button behind the brake pedal is activated when you press down on the brake pedal and makes the brake light go on. Press the switch by hand to see if it's stuck. If it is, pressing it may force the stuck part.
Find the fuse for the brake light and pull it out. Remember to put it back in before driving again
Find the brake lightbulbs and remove them. Remember to put them back in before driving again.
Disconnect the negative battery cable. You will have to reconnect it again to start and drive your car.
If that doesn't work, try a new relay switch.