Shut off the water first
Find the main shut-off valve. It is usually where the water line enters the house, near the meter, in a basement, garage or utility cupboard. Turn it clockwise until it stops.
Make the area safe
If water is near outlets, appliances or the electrical panel, switch off power to that area at the breaker, but only if you can reach the panel without standing in water.
Call, then document
Call a licensed plumber, then take photos of the damage for your insurer. Move valuables and open cabinets so wet areas can start to dry.
This is general information for a sample site, not professional advice.