First turn off the water to the toilet. Most of the time you will find a shut off valve on the lower left side of the toilet. Turn this valve to the right (clockwise) to turn it off, and left to turn it on. To be sure it is off, flush the toilet, if the toilet does not start to re fill the tank, then the water is off. If water is going back into the tank and you have turned the valve as far to the right as you can, you will need to turn off the main water valve. f you can not turn this valve with your hand, just turn off your main water valve With the water off remove the supply line from the bottom of the toilet. Instructions on how to replace a toilet fill valve or ball cock Toilet fill valve, Fluid master, ball cock, running toilet, If you are not completely sure you can do this job, maybe you should have it done by a professional.