Take the lid off the cistern - flush it - see the water trickle back in and lift a float - see the float arm bear against the inlet valve slowly turning off the water - pull up hard on the arm and notice the water stopping - push down and see it coming in again.
If there's a nylon adjuster screw (with a locknut) screw it inwards towards the valve - else adjust the float to be lower in the water so it rises up sooner and shuts the water off quicker.
You will see the overflow tube - try and adjust the water level to be at least half an inch or so lower.
