The answer depends on the valve. In the USA, the normal direction is clockwise (i.e. in the direction that the hands of an analog clock move).
shutting off the valve
Behind and/or under the toilet is a shut off valve. You only need to turn it like turning the faucet off. If that doesn't work, you'll have to turn off the main valve located outside the house in some cases.
Shut the water off at the main shut off where it comes into the house or at the meter and change the valve at the fixture. The valve at the fixture may be sweated on, compression, or threaded.
Make sure your main water is turned on by turning on another sink or bath and if it is then turn your toilet valve off and on a couple times and see if any water comes out in your tank. If still no water is filling the tank then shut off the valve and take the supply tube off the valve and turn the valve on slowly to see if you have any water comming out. If still no water then your valve is plugged or your pipe to the valve is plugged which then you would need to clean out your valve or lines with the whole house water shut off. Sometimes I have seen that the plug is in between the valve an supply pipe connection and as soon i take the supply pipe off and turn on the valve it works just fine even when I put the supply pipe back to the toilet. I don't think it would be in your fill valve in your toilet tank unless you have pressure assist tanks and with them all you need to do is take out the screen inside them.
It does not increase the pressure it remains the same as if the valve was open.
Shut off valve at the wall? Turn the main water line off and change it. How you change it would depend on what type it is and how it attaches to the pipe.
Refill valve is a very important component in your toilet since it fills the tank of the toilet after every flush. This valve uses a float to turn the water on and off. This is a plumber related work, only a professional plumber can repair it. alliedallcityinc.com
because the water is turn off
your water pressure/volume is to much for the overflow to handle.after repair try to turn shut off valve under toilet to slow water fill in tank.test by holding down float while refilling toilet.
It's something wrong with the pipes! One pipe is on too tight so it whistles when flushing. Here is THE answer!!! I did it to my own toilet. A plumber came over to fix our stuck toilet... The first thing he did was turn off the water supply valve at the back of the toilet. Then he unstuck our toilet. When he left, he turned the water supply valve back on, but he turned it up to high. the water pushes the air up through the pipe and the pressurized air comes out the top little slits on the stopper thingamagigie. To put in words you can understand, it's like blowing a flute. The Fast fix is to do this... Turn the water valve one way and then another. You will know when the water flows faster because the whistling will still be there. What you want to do is SLOWLY turn the water valve to the off position UNTIL the whistling stops and then a little more. You now have water coming in, but not at a speed to pressurize the air and make the whistling sound!
The washer in the shut off valve has probably split or come off completely over time.
Have the new valve in hand. Cut the toilet water supply off at the commode, or even at the street meter box.Remove water tube or pipe supply and old flush valve. Install new valve and new supply line if required.