If you need to stop your water supply your need to find your inside stopcock / stop valve. Once you have found it turn the valve clockwise. It may take a few turns to stop the water but you'll usually get to a point where you can’t turn it anymore. You shouldn’t need to use excessive force to turn it further. It may also take a few minutes for the water to stop running from your taps, as there will be some water left in the pipes from before you turned the water off.
The stopcock is usually inside your house and normally located just after the water pipe enters the house. This is often under the kitchen sink, but can also be:
In an airing cupboard
Under the stairs
Under the floorboards
Near the front door
In the garage, utility room, bathroom or cellar
For flats, you may have a communal inside stop valve if your water supply is shared with your neighbours. This is normally located where your water supply enters the building. Sometimes an additional inside stop valve can be found where the water supply enters your flat.
Remember though, if you are unsure then it is always best to call a Checkatrade vetted plumber.