Main valves are commonly found where the service line enters the house, in a basement or crawlspace, garage, utility area, or exterior meter box. Identify yours before you need it, keep the area accessible, and label it. Do not force a seized valve; an older valve can break when handled aggressively.
Start with what you can observe safely
Useful details include which fixtures are affected, whether the symptom is constant or intermittent, and what changed just before it began. Photos can help document visible water or corrosion. Never enter a flooded area near electrical equipment, force a stuck valve, or dismantle pressurized plumbing.
When to call a plumber
Professional help is appropriate when water cannot be controlled, a problem returns after a basic cleaning or reset, several fixtures are affected, finishes are becoming wet, or you are unsure whether continued use is safe. Early attention often preserves more repair options.