Inlet valve letting too much water. For solve this, change the inlet valve or the ball float.
C-pack is not sealing. To solve this, it will require either for the c-pack washer or whole c-pack to be replaced.
Toilet pan is cracked. A toilet pan will need to be replaced.
Flexible hose leaking. The flexible hose will need to be replaced, remember to turn off your water supply beforehand.
Pan connector leaking. Wax sealer or a foxy fit pan connector is required to solve this problem.
Angle valve is leaking. The angle valve or shut off valve must be changed.
These may be done by a DIY enthusiast. But, if you are not familiar it is best to call a registered plumber.