Once you have turned off the water supply to the toilet at the shut-off valve, the first thing you should check is the packing nut. It is located just below the handle on the stop valve. The packing nut holds the packing tight against the stem of the handle. This packing, which is normally comprised of a compressive or graphite material, is the only thing that is keeping the water inside the faucet. When the handle is turned, the packing can start to leak, and you will notice water coming from the stem. Try turning the packing nut a quarter turn with a wrench. This should compress the packing washer so that it creates a tight seal against the stem. Be careful not to actually remove the valve; you only want to tighten the packing nut. If this does not stop your leak, you should call a professional.