When you look at the master cylinder you should be able to see how much fluid you have in it. There are two marks on the body. One is Low and one is Normal (if I remember it right). If the fluid is at the Low mark you will have to buy brake fluid in any automotive supply store. Make sure you verify the correct type. As far as I know most of the cars (including yours) use DOT3. Add fluid.
By the way low fluid level could trigger the brake warning light.

Sam