When water is spraying out of a place it shouldn’t, you need to shut the water off right away. Some fixtures have their own shutoffs–usually a round knob that you turn to the right–but many don’t. If the water is coming out of a leak in a pipe, you have no choice. You need to turn off the water to the whole house. Fast!
Plan ahead
If you take a little time now to find where the main shutoff is you’ll save valuable time when you have to shut the water off quickly before it ruins the floor, the cabinets, etc.
There is a main shutoff right next to the water meter in the ground near the street if your house is in a city or suburb in a moderate climate. (In colder climates, like, I don’t know, Minnesota, your shutoff is inside the house in the basement. See below.) Look for a metal cover in the ground that looks like this.

Or this.

Pry up the cover (you might need a screwdriver).
Inside you will see the water meter, which has an “odometer” for how much water you’ve used, and the main shutoff.

When the metal bar on the top of the shutoff is pointing down the pipe, like in the photo above, the valve is open and water flows. If the bar is perpendicular to the pipe, the valve is shut. Think of that bar as being in the pipe itself. If it is crosswise it will block the water.

There is a special tool for turning the shutoff valve that you or your neighbor might have. If not, use an adjustable wrench or a pipe wrench. The valve will turn only a quarter turn.
These valves can be hard to turn. You might need to tap the end of the wrench with a hammer to get it moving.
Turn it clockwise to close it and shut off the water. Counter-clockwise to open it and let the water through.
Around the globe
Some houses will have a main shutoff knob in the basement next to a meter that you turn like a typical outdoor hose faucet. These are called “globe valves.” Keep turning to the right to turn off the water.


Make it better

A better setup is to have a separate in-line shutoff closer to the house
or inside the house. The one shown in the picture is a simple lever model, which is called a “ball valve”, that works as described above. In this position the water is on.
To shut off the water, turn the lever a quarter turn so that it is perpendicular to the pipe as shown in the next photo.

If you don’t have one, I recommend installing a lever-type main shutoff in the basement or crawlspace that is easily accessible. This type is very easy to open and close and it clearly shows if the water is on or off.