Where to look
Often where the water line enters the house: garage, utility room, basement, or near the water heater. Some homes have it in an outdoor box near the street.
Test it
Turn it off and on once a year so it doesn't seize. Label it so everyone in the house can find it.
In an emergency
Shut off the main, open a low faucet to drain pressure, and call a plumber.
General information only, not legal or professional advice.