Stop the source. Then breathe.
If water is actively flowing, stopping it comes before everything else. Documentation does not matter if the loss is still expanding. Take a breath and locate the source.
Plumbing failure
Shut off the main water supply. The main valve is typically in the basement near where the water service enters the home, often on the wall facing the street. On Staten Island homes built between 1950 and 1990, look for a brass or chrome gate valve about 6-12 inches up from the floor. Turn clockwise until firm. If the valve is stuck or you cannot find it, call NYC DEP at 311 for emergency shutoff at the street.
Appliance failure
Unplug the appliance immediately if it is safe to do so. Most appliance leaks have their own shutoff valves nearby — under the sink for the dishwasher, behind the washer for the laundry valves, behind the refrigerator for the ice maker. Use those before reaching for the main.
From above (ceiling leak)
If water is coming from a ceiling, the source is the floor above. Get to that floor and find the source — usually a burst supply line, leaking toilet, overflowing bathtub, or failed appliance. If you live in a multi-family or apartment building, this could be the unit above yours. Contact the building manager or that tenant immediately.
Sewage backup
This is a Category 3 black water event per IICRC S500. Do not attempt cleanup yourself. Evacuate pets, isolate the area, do not let anyone walk through it without protective footwear. Call NYC DEP at 311 to report. Call your insurance carrier immediately. Call a professional restoration contractor for emergency response.