Do what they said...check your fluid level. If it's low, checks the pads. As the pads wear down, the calipers have to expand out further, in turn causing the fluid level in the resevoir on the master cylinder to drop, eventually causing the brake light to come on. Also, if the pads are low, don't add more fluid just to turn the light off. Replace the pads and then adjust your fluid level accordingly.

If none of this seems to be your problem, start checking your calipers, wheel cylinders (I'd assume you have disc brakes all around, but if not, you'll need to check these in the rear), master cylinder, and hoses for leaks.