Undo the cable at the brake. Pull on the lever while holding the cable in your other hand. This will immediately tell you if it's a cable/housing problem. If it feels smooth and there's no drag check the caliper itself by squeezing the arms together, they should snap back quickly and smoothly. If that's ok it might be a pad wear problem like
cyccommute posted. It's not hard to figure any of this out if you just look at it logically. Break the system down into its separate parts...lever, cable/housing, caliper/pads.