The main shut-off is usually where the water line enters the home, often near the water heater or an outside wall. Knowing its location before an emergency saves the most damage. Turning it off stops new water, though it does not reverse what is already soaked in.
Safety comes before belongings. If water is near outlets, the panel, or appliances, cut power at the breaker before wading in. Do not use a household vacuum on standing water.
Photograph and video everything before you move or clean anything - the standing water, the source, and the damaged contents. That documentation protects your insurance claim. Then call for professional extraction; wet materials keep absorbing water into cavities and subfloor by the hour.