Find the shutoff before you need it.
Every emergency plumbing call gets shorter and cheaper if you already know where this is. It takes ten minutes on a dry afternoon.
Inside the house
Follow the cold supply back from the water heater; the main valve is usually upstream of it. In homes here that commonly means a garage wall, a utility or laundry room, a crawlspace hatch, or the basement wall on the street-facing side.
You are looking for either a round wheel handle, which closes clockwise and may take several turns, or a lever, which closes with a quarter turn perpendicular to the pipe.
At the street
If there is no interior valve, there is a meter box in the ground near the curb with a lid you can lift. Inside is the meter and a valve on the street side. A meter key is a few dollars and works far better than a screwdriver and optimism.
Test it, then label it
Close it, open a faucet at the lowest point in the house and confirm the flow stops. A valve that has not been turned in fifteen years sometimes will not, and finding that out now is enormously better than finding out during a burst. Then label it — a luggage tag on the handle is enough — so that anyone in the house can find it.
Common questions
My shutoff valve will not turn. What now?
Do not force it — an old gate valve can shear or fail open. It is a small scheduled job to replace it with a quarter-turn ball valve, and it is the cheapest insurance in plumbing.
Tell us what the house is doing