Is the brake fluid level where it's supposed to be?

Other than that, the first (and easiest) thing to do is bleed your brakes, like USC said. While you're down there, see if you notice any brake fluid on the calipers -- they should all be bone dry. If you see any fluid, you've got a leak.

If bleeding them doesn't fix it, then you've got a leak somewhere. If the calipers are dry, I'd suspect the master cylinder. When they leak externally, you'll usually be able to see some fluid behind it, near the booster. If EVERYTHING is dry, and you're not losing any brake fluid, and the pedal is still going down too far, it's probably an INTERNAL leak in the master cylinder.