If water slowly dribbles into the bowl after filling then the float valve needs to be adjusted - the float needs to placed lower into the water to make the inlet valve shut sooner - a half inch below the overflow tube is good.
If the water leaks during flushing from the joint between the cistern and the toilet pan then there's an adjustment you can make - I could tell you how but I'm too lazy. Youtube has videos all about the donut washer...
https://www.youtube.com/watch?v=Wj-5YXnY-_Y
There's a squishy washer between the two units and a plate to squash it tight - two wing nuts pull down on the plate - right at the back of the pan - one each side - just do them up.
You might need a new donut washer
Good Luck...
March 2017