Locate the water supply valve by tracing the hose from the bottom of the tank to the wall.
Turn the valve clockwise as far as it can go to stop the flow of water.
The valve should be relatively easy to turn so don't force it if it feels stuck.
You might be dealing with rust or calcification. Try twisting it side to side gently. If needed, use white vinegar to dissolve the rust/mineral build-up.