Turn off the water supply to the toilet
Remove the tank lid
Check the flapper for wear, warping, or poor sealing
Replace the flapper if it is damaged
Make sure the chain has slight slack and is not too tight
Adjust the float so the water level stops below the overflow tube
Inspect the fill valve for damage or continuous flow
Clean or replace the fill valve if needed
Check the flush handle for sticking or misalignment
Tighten or replace loose tank bolts or internal parts
Inspect the overflow tube for cracks or improper water level
Turn the water supply back on and test the toilet
Replace the entire fill valve or flush assembly if the problem continues