Turn off the water supply valve behind the toilet
Remove the tank lid
Check the flapper for wear, warping, or debris
Clean the flapper and flush valve seat
Replace the flapper if it does not seal properly
Check the chain length and adjust it so it is not too tight or too loose
Inspect the fill valve for damage or malfunction
Adjust the float if the water level is too high
Make sure the water level is below the overflow tube
Tighten loose tank bolts or connections if needed
Replace the fill valve if it keeps running
Turn the water supply back on and test the toilet
Replace the flush valve or call a plumber if the problem continues