There's three things that turn the light on. First check the brake fluid level in the reservoir. Be absolutely certain to not get an petroleum product like oil or power steering fluid in there. That will result in a real expensive repair. If the fluid is low, there's either a leak or the front disc brakes are about worn out. The fluid should not be filled if there's no leak. The level will come back up when the new pads are installed.
Be sure the parking brake is fully released. If the pedal or handle stays partially engaged, suspect stretched or rusted parking brake cables.
A leak in one of the two hydraulic systems will turn on the warning light. Look for puddles on the ground, wetness on the inside of any of the tires, or wetness behind the master cylinder.