rj713 makes a really good point: be sure to get it fixed. A soft brake pedal like that is a serious problem, and is indicative of a leak in a brake line. You will be able to add brake fluid on your own without any problems. Go to your local auto parts store and ask them for the right brake fluid for your car (probably DOT 3 brake fluid). As per the probable leak, take it to a mechanic and have him/her do a thorough inspection of the brake system and make the necessary repairs. This should be about what your engine compartment looks like. I don't have exact info, but the master cylinder is most likely the amber/clear reservoir with a black cap in the far right of the image I provided.
Until you can get more brake fluid in and the car in for service, pumping the pedal a time or two should build up pressure in the system thereby raising the pedal, but only for a few seconds.