hedpe - under your hood, against the firewall, just in front of the driver's side you'll see a large plastic circular contraption know as the brake booster. Sticking out from its center toward the front of the car is the mastercylinder. You can't miss it because it has the brake fluid reservoir smack on top of it. Its usually silver in color.

You might also want to Check the fluid in the reservoir and also check to make sure you have no brake fluid leaks just behind your wheels signifying a ruptured brake line.