The initial step is to determine the cause of the leak. There are two different components that could be the problem, the fill valve or the flapper. Remove the toilet tank lid and place it in a safe flat place so it won't get broken. If you notice that water runs constantly into the tank and into the overflow tube, you most likely have a fill valve that either needs adjustment or replacement. If the water intermittently runs but does not flow into the overflow, you probably need a new flapper. First, let's look at a fill valve issue. Here's an easy check.