How do I stop a running toilet without calling a plumber?

The most common cause is a faulty flapper at the bottom of the tank. Turn off the water supply valve behind the toilet, flush to empty the tank, then unhook the old flapper and take it to a hardware store to find a matching replacement. Snap the new one onto the overflow tube and reconnect the chain, leaving a little slack. Turn the water back on and test it. If that doesn't fix it, the float may be set too high, causing water to spill into the overflow tube. Bend the float arm down slightly or adjust the float screw until the water stops about an inch below the top of the overflow tube. Both fixes cost under $10 and take about 15 minutes.