Breaks And Master Cylinder

There could be a few things causing spongy brakes. The first thing to check is the brake fluid level in the master cylinder. The master cylinder is in the engine compartment. It's on the driver's side firewall and should have a cap that has DOT3 or something similar on top. Make sure the fluid is up to the "full" line in the brake reservoir. If it's low, add brake fluid up to the full line.

You could also have a brake hose, wheel cylinder or a brake line leaking fluid. You can spot that by the fluid leaking onto the ground where you park. A leaking wheel cylinder, brake hose or brake line is a serious safety hazard and should be repaired as soon as possible.

You may also have a bad master cylinder. Unless you are a good auto mechanic, that problem is difficult to diagnose by yourself. There may be some brake fluid on the driver's side floor (under the dash), but that's not always the case. A bad master cylinder is also a potentially serious safety hazard, so it would need to be replaced immediately. 

air in the breaks will do this. 

also if you pads are very low that also might contribute to a spongy pedal