The first thing to do is find the place in the street where you can turn-off your water. It's likely to be about 3 feet down under a metal lid and you will need a special tool to turn it (a piece of wood with a V cut out will probably do). The first time I came to use ours it was full of sand and I had to dig down to get to the tap. Turn the water off there, check that it's really off ie, that you've found the right tap. Then, and only then, try a spanner on the one inside the house. Once you've finished the job consider replacing the stop-cock inside the house - you might need it urgently sometime.