Find the house valve
On most homes here it is on the exterior wall closest to the street, often within a few feet of the front hose bib, sometimes inside a small access panel in the garage.
A round handle turns clockwise several full turns to close. A lever style ball valve turns a quarter turn. Either way, close it and open a tap to confirm the water actually stops, then reopen it slowly.
Find the meter valve as a backup
The meter sits in a concrete box near the sidewalk. Under the lid, the valve on the street side of the meter needs a meter key or an adjustable wrench on the flat tab. Turn it a quarter turn until the tab is crosswise to the pipe.
Keep a meter key with your tools. Trying to buy one during a flood is not a plan.
Local shut-offs worth knowing
Each toilet has a small valve behind it. Each sink has two under the cabinet. The water heater has one on the cold inlet. Closing the fixture valve instead of the whole house keeps the rest of the home in water while you wait for a repair.
Old multi-turn valves often seize or start weeping the first time they are used. If yours will not move, or drips after you close it, replacing it is a small job now and an expensive one during an emergency.
After the water is off
Open the lowest tap in the house to drain pressure out of the pipes, then the highest one to let air in. That stops water still in the lines from continuing to pour out of the break.
Photograph everything wet before you clean up. Insurance will ask.
Need a hand with this? See our emergency plumbing repair page, or call 626-386-9105.