Turn off the water at the main.
Pull out the split pin where the ball valve connects to the water inlet in the cistern.
You can then remove the arm. As you do this you can slide out the litlle piston that it keeps in place.
You can see a rubber washer at one end (this is probably hard and causes your leak)and needs replacing. The piston unscrews to let you replace this washed (get washer from hardware store before you begin). Having replaced the washer just reverse the dismantling and turn the water back on,,,job done---or call a plumber. John
John Somers
March 2011