Very simply stated, you problem is that your float valve is adjusted to shut off at too high a level of water in the tank. This allows the tank level to rise above the overflow tube, thence to drain into the bowl and into your toilet drain pipe. This can waste a lot of water and cost you mucho bucks. As RP suggests, you need to adjust your float valve to shut off at a level about 3/4″ below the top of the overflow tube. There are various ways to do that, depending on the particular design of the float valve.