First, remove the toilet tank lid and lift the float or fill valve to stop the water from refilling the tank. Then, turn off the water supply to the toilet – this is usually the pipe at the bottom of the toilet, and the valve is usually on the pipe itself. Finally, turn off your home’s main water and assess the issue. If the water continues to overflow after turning off the main water, the issue could be unusually serious. Either way, it’s best to call a plumber to get to the root cause of the issue after an overflow issue.