How do I fix a running toilet myself?

The most common cause of a running toilet is a faulty flapper. Turn off the water supply valve behind the toilet, flush to empty the tank, then inspect the flapper at the bottom of the tank. If it looks warped, cracked, or has mineral buildup, replace it. Flappers cost about $5 at any hardware store and simply unhook from the overflow tube and disconnect from the chain. If replacing the flapper doesn't stop it, the fill valve may be the issue. You can replace the entire fill valve for around $10-15. Also check that the float is set to the correct water level, about an inch below the overflow tube. Both repairs require no special tools and take under 30 minutes.