One of the most common causes for a running toilet is water leaking into the bowl via the overflow tube. This will happen when you have too much water in the tank. It is easy to adjust the water level by adjusting the height of the float.
To lower the water in a toilet with a float arm, loosen or tighten the screw until the float arm is lowered.
It is easy to lower the water in a toilet with a column float attached to the fill valve. Loosen the screw or clip, push down the float, and tighten everything back up again.