Right now

Urgent plumbing help

Damage is a function of time. Do these in order, then call.

Stop the water
Closest valve first, main second. A fixture that is overflowing has an angle stop underneath it — turn it clockwise. If you cannot identify the source, go to the main.
Deal with electricity before water
If water is contacting outlets, light fixtures, appliances or a panel, do not enter it. Shut off power to that area at the breaker only if you can reach the panel safely and it is dry. If you cannot, stay out and call an electrician or the fire department.
Know what is not a plumbing call
Suspected gas: leave the building, use nothing that sparks, call the gas utility from outside. Sewage in living space: keep people and pets out — it is a biological hazard. A sagging or bulging ceiling: stay out from under it.
Limit the damage
Once the water is off and the area is safe, move what you can, get standing water up and open the space to air. Photograph everything before you move it.
Then call
Have ready: what happened, what you already shut off, whether the water is stopped now, the property address, and who else is affected.

Call rather than filling in a form.

A form cannot ask you the follow-up question that matters.

If there is a fire, a suspected gas leak, or anyone is injured, call 911 first.