Float ball is too low or adjust water-intake assembly
Reposition the Float Ball
If the float ball is too low the toilet may not flush completely and you may need to adjust the float ball back up slightly to get more water in the tank. You can simply bend the float arm up slightly to keep the water about 1 inch below the top of the overflow pipe. Flush the toilet to a make sure it enough water is getting into the bowl.
If it also possible that the float ball is damaged or rubbing against the side of the tank. If it is rubbing the tank, gently bend it toward the center of the tank. If the float ball is damaged, it will not rise enough to close the ball cock. The float ball can be screwed off of the float arm counter clockwise. You may need to use pliers to grasp the float arm. When placing the new float ball on the arm, apply petroleum jelly or plumbers tape to the threads.
Adjust a Water-Intake Assembly
Your toilet may have a water-intake assembly instead of a float ball. To adjust the water level in the bowl so it does not flow into the overflow pipe, pinch the clip attached to the thin metal rod and slide it and the cup down to lower the water level. Sliding the clip and cup up will raise the water level. Try moving the clip about an inch at a time.
Ball cock is damaged
The ball cock opens and closes as the float ball falls and rises with the water level in the tank. This action controls the flow of water into the tank. With a plunger-style ball cock the float arm pushes the valve plunger and washer to stop the water flow. With a diagphragm-style ball cock the plunger pushes against a rubber diaphragm.
Before working on the ball cock turn of the water and flush the toilet to empty the bowl. Remove the screws surrounding the ball cock assembly to gain access to the washers or diaphragm. You may need to slide the float arm out of the way to gain access to the assembly.
Remove the washers or diaphragm from the valve plunger with a small screwdriver. Use vinegar and a small brush to clean sediment from inside the ball cock and washer or diaphragm. Once cleaned reassemble the parts and the ball cock. If any of the pieces appears damaged or if the water continues to run, replace the ball cock.