A toilet fill valve can come in many different versions. Two main versions include a ballcock version that was prevalent in toilets years ago. This type of filling mechanism was a large black float ball on a metal or plastic rod. When the water caused the float to go up to a specific level the lever would cause the valve to shut off. Another style, more common today does not have the ballcock. It is a mechanism that feels the water level and stops the inflow from the valve by a float that rides up the fill tube of the toilet.
Step 1 - Familiarize Yourself with Your Toilet Float System
Open the top of the toilet tank. Be careful that you do not drop the tank cover, it is heavy and ceramic, it will break if dropped. Look inside the toilet tank to determine which type of fill valve you have. You will either see the large black ballcock float, about 5 inches long by 4 to 5 inches in diameter. It will be attached to the end of a lever. If you do not see a ballcock assembly, you should see a tube coming up from the bottom of the tank with a black or blue assembly that rides the fill tube.
Step 2 - Investigate the Valve
Put on the gloves to protect your hands from potential cuts. There are sharp objects in the toilet tank. Flush the toilet and watch how the water is filling into the tank. Make sure that the devices inside the tank are doing their job. Ensure the flapper is closing properly and see if the float is doing its job. If you find the tank does not stop filling, lift the float and see if the water stops running. If it does then an adjustment of the float will fix your problem.
Step 3 - Adjust Your Float and Fill Valve Assembly
You have to keep the water turned on for this process to ensure you are stopping the water flow as you adjust.
If you have the newer toilet float system, one that has the float on the fill valve tube, you can pinch the contacts on the sides of the float and move it up and down the tube until the water shuts off.
The ballcock float connects to a lever that is held to the valve by a screw. Adjust the height of the ballcock float by tightening or loosening the screw. If for some reason the screw is maxed out, you can adjust the ballcock float by bending the lever. If you choose to bend the lever, be sure to remove it from the valve first because if you accidentally break the valve you will be in bad shape.
Step 4 - Verify Adjustment
Verifying that your adjustment is adequate is as simple as flushing your toilet. Ensure that the water fills to the level you wish and that the water flow then shuts off. Put the cover back on the toilet and keep an eye on the tank fill for a day or two to ensure satisfaction.