If water runs continuously, remove the tank lid and check the water level. If water is running over the top of the overflow tube, lower the level. If there is a float ball, bend its rod so the ball sits lower.
To adjust the water level when there is a float-cup assembly, pinch the spring clip and slide the float cup up or down -- up to raise the water level, down to lower it.
A kinked or tangled chain may cause the water to run on. Detach it from the handle arm, undo the kink or tangle, and reattach it so there is about 1/2 inch of slack in the chain when the flapper is seated.
The lift wire attached to a stopper must slide smoothly. You may need to bend the wire that connects the lift wire to the handle to straighten any kinks. Work carefully and test thoroughly until the lift wire slides without interruption.
If these measures do not solve the problem, the fill valve needs to be repaired or replaced. The tank must be emptied for this and other repairs. Shut off water to the toilet and flush the toilet to remove most of the water. Use a sponge to soak up the rest of the water.