If water leaks between the toilet tank and bowl whenever you flush your toilet, you need a new tank to bowl gasket. Often referred to as a spud washer. No matter what type your toilet is, use the pink colored Crane gasket. It will fit most models and is the best one by far, in my opinion. To repair the leaking you have to replace the gasket you must disconnect the water line to the toilet and remove the tank from the bowl by removing the two brass bolts holding the tank and bowl together. You must use new bolt gaskets when re-assembling. See diagram of tank-to-bowl gasket.
This is the Crane tank-to-bowl gasket. It is by far the best shape of all the tank-to-bowl (spud) gaskets. This gasket will fit most any flush-valve on most makes and models. Use it if you can. The only time you need to change a tank to bowl gasket is when the toilet leaks between the tank and bowl during flushing, or if you need to remove the tank for some other reason. Never use the old one, it's not worth it!