Step one: stop the water
Every fixture has a local shut-off — the small valve under a sink or behind a toilet. Use it if the problem is confined to one fixture.
For anything larger, go straight to the main shut-off. In most Davis County homes it is in the basement or crawlspace on the wall facing the street, often near the water heater. Find it today and hang a tag on it, because looking for it while water is spreading is how basements get ruined.
Step two: kill power if water is near electrical
Water reaching outlets, a furnace, or a panel means shutting off the affected circuits at the breaker — but only if you can reach the panel without standing in water. If you cannot, leave the house and call the utility.
Step three: contain and document
Towels and buckets slow the spread while you wait. Move furniture and anything absorbent off wet flooring.
Take photos and short videos before you clean up. Insurance adjusters want to see the source and the extent, and your memory of it will not be enough.
Special cases
Sewage backup: stop using every drain in the house immediately, including toilets and the washing machine. Each additional gallon has nowhere to go but up through the lowest opening. Keep people and pets out of the affected area.
Smell of gas near a water heater or gas line: do not touch light switches, do not use your phone indoors. Leave the house, then call the gas utility from outside. Plumbing comes second.
No water at all in freezing weather: shut off the main before you start thawing. If a pipe has already split, you want the water off before the ice melts.