Turn off the water supply valve behind the toilet
Remove the tank lid
Check the flapper for wear, warping, or debris
Clean or replace the flapper if needed
Make sure the flapper chain has a little slack
Adjust the chain if it is too tight or too loose
Check the fill valve for proper operation
Adjust the float so the water stops at the correct level
Replace the fill valve if it keeps running
Inspect the overflow tube for water spilling into it
Lower the water level if it is too high
Tighten any loose tank bolts or connections
Replace worn tank-to-bowl or supply line parts
Turn the water back on and test the toilet
Replace the flush valve assembly if the problem continues