Before you start, turn off the water at the shutoff valve near the base of the toilet. If there isn't one, read "Emergency Shutoff, " which will fill you in on how to locate shutoff valves. Once the water is off, flush the toilet a couple of times. This empties the tank so you can make repairs. If you're replacing the water-supply line or ball cock, sponge up the remaining water in the toilet tank. Next, check out your toilet-tank mechanism and match it to one shown on the following page before buying parts or reading up on the repairs that we describe. When buying parts, choose a kit (less than $20) rather than individual items. The kit will contain all of the working parts you need to repair your toilet. Buying a kit will also eliminate unnecessary trips to the hardware store or home center in the middle of the job.