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Start by jiggling the toilet handle. If this stops the running adjust the lift chain inside the tank. The chain "may" be wrapped around the lift arm and you may need to move the position of the chain on the arm.
Simply disconnect the hook from its slot and reconnect at the next slot on the float arm, and if this doesn't work check to see if the float has become disconnected. The top of the float should be slightly above the water's level. If it's not adjust the arm on the float until the float rests where the water level is about 1/2" below the top of the overflow pipe.
If the toilet is still flowing freely, try using a dye tablet (available at hardware of plumbing supply stores). Simply flush and as the water in the tank begins to fill, drop in the tablet. If your tank is leaking you'll begin to see the water in the bowl turn the color of the tablet.
Sometimes the flapper valve is the culprit and doesn't seal the tank properly. This should be easy to fix. Turn off the shut-off valve below the tank and flush the toilet to "empty" it. Remove the flapper and replace it with a new one. If the flapper valve seat is cracked, you'll have to replace the whole unit, which you can find at a hardware store. Be sure to follow the manufacturer's instructions carefully.