97hoopty

Have a 1997 Corolla DX, had brakes done a year ago. All of a sudden, the brake light is lit on the dashboard, and I recently had to have a battery replaced b/c the lights were stuck on and I failed to see it. The brake light goes on and off, making brake lights go on and off constantly. What could possibly be wrong before I head to a mechanic? Is it maybe a fuse, or that I need brake fluid. I'm not the car's original owner, no manual in the car; by the way, where would I find the reserve for the brake fluid?