The water level in the tank should be at least 1 inch below the top of the overflow tube. If the water level is at the top of the overflow tube, this could be where a leak is occur ring. The float that controls the water level in the tank should be ad justed so that the water level in the tank is at least 1 inch below the top of the tube. Most toilet tank leaks result from worn parts or misalignment of a part of the flushing mechanism. If you don't feel comfortable repairing it, call a plumber.
Water leaks are costly and an unnecessary waste of water.