Whenever water is being used anywhere in the house it backs up in the main floor toilet. The water is clean. What steps should I take now?

Whenever water is being used anywhere in the house it backs up in the main floor toilet. The water is clean. What steps should I take now?
City sewer or septic tank? Locate your house cleanout and open it up. Is there standing water in the line? If so the blockage is downstream from the cleanout. If the pipe's dry the clog's in the house system and must be snaked from a roof vrnt or a pulled toilet. Good luck, Tom