Water running on and on into your toilet tank is both irritating and wasteful, but it’s usually easy to fix. If water runs constantly, it could be draining into your overflow pipe, which you can see by lifting the tank lid. Lift the float ball (a plastic ball attached to a horizontal arm) to see if it stops, and adjust the level of the ball with the adjustment screw or by simply bending the arm so it sits lower. If this doesn’t work, unscrew the float ball and shake it to see if there’s water in it. If there is, replace it. If the water doesn’t stop when you lift the ball, but you feel resistance, the intake valve needs a new washer or should be completely replaced with a new plastic flush assembly.
Running water could also be caused by a leaking or misaligned tank ball or flapper. These are the rubber stops which allow water into the toilet bowl when flushing. Drain the toilet by shutting off the supply and flushing, and realign as necessary by adjusting the chain or lift wire. Clean the area in which the stop sits by lightly buffing with fine steel wool, and replace worn or corroded parts. You can replace a tank ball (held by an arm to the flushing lever) with a flapper; just attach the flapper chain to the lever, and clip the flapper itself around the base of the overflow pipe.