YOU HAVE TOO MUCH WATER IN THE CISTERN NOT TOO LITTLE - IT IS OVERFLOWING
Take the lid off the cistern
Flush the loo and watch what happens - several times if necessary.
A float of some sort will fall down as the water level drops
As the float rises with the incoming water, an arm will bear against the water inlet valve and slowly turn off the flow - lift the floaty thing by hand - pull it up as far as it will go and observe how it shuts off the water.
Sometimes you get too much water in the cistern because the valve doesn't shut off properly - water continues to enter the cistern and overflows (used to be outside the house on the really old cisterns but the more modern ones now overflow into the pan).
There are two ways to make the valve shut sooner - adjust the float to be lower down in the water (means it rises up more and closes the valve sooner) - OR - adjust the little nylon screw thingy that operates the shut-off valve (there's usually a lock nut on the screw which you will have to undo a bit to make your adjustment) screw the adjuster in a few turns and operate the flush - see how the water level changes when it shuts off - when you're content do up the lock nut.
Remember you need to lower the water level to stop the overflow - about half an inch below the overflow pipe should do.
If it's like this just bend the arm down...
http://www.diydata.com/plumbing/ball_valves/float_valve.gif
Here's a really sexy one where you screw the blue float up and down to adjust it...
http://img.alibaba.com/photo/52195531/float_valve.jpg
Some look like this...
http://images.productserve.com/preview/1228/14398295.jpg
Good luck...
Peccavi
March 2009