Job one — stop the water. The most valuable thing anyone in the building can do, and the one everything else waits on. Where: the fixture stop if you can identify which fixture is involved — the small valve on the supply line under a sink or behind a toilet, turned clockwise. The main shutoff if you can't, if the leak is inside a wall or ceiling, or if the fixture stop won't hold. Then open a low tap — a ground-floor or basement one — to drain the pipes above the failure. It reduces what continues to escape while you wait. If it's a drain backing up rather than a supply leak, the equivalent action is stop using water entirely — every flush and appliance cycle adds to what's coming back.