Use food coloring to check for leaks: Remove the toilet tank lid. Put several drops of food coloring in the tank. Replace the lid but don't flush the toilet. After 10 minutes, look in the bowl. If you see color in the bowl you have a leak, probably from a worn flapper.
Remove the flapper: Disconnect the chain from the flush arm and unclip the flapper from the overflow tube. There are many different flappers on the market, so bring your old flapper with you to the store to ensure you get the correct one.
Install a new flapper: Clip the new flapper onto the chain and reconnect the chain to the flush arm. Flush the toilet and make sure the new flapper properly seals the water inside the tank.
Consider an upgrade: If your toilet was made before 2004, replacing it with a high-efficiency WaterSense or Premium WaterSense-labeled toilet can save a homeowner up to $200 per year on utility bills and reduce water waste. You might also be eligible for a rebate.