If the toilet is running continuously, then the problem is the fill valve. The fill valve has moving parts in it and can wear out over time. It is susceptable to problems with debris or chlorine that may be present in the water. Inlet pressure in the water supply can also cause issues.
The remedy is to clean any screens or diaphragms inside the fill valve, or replace it with a new one. If your water pressure coming into the house exceeds 80psi, it will need to be reduced.
If the toilet is running occasionally, then the problem is the flapper.
The remedy is to first check to see if there is any grit that could be keeping the flapper from sealing properly on the flush valve. Simply wipe the bottom of the flapper and the area around the flush valve with a soft cloth. If the problem persists, then replace the flapper.
Bring the original flapper or fill valve into Bay Plumbing Supply so we can immediately know which one will be best to replace it with. Knowing the make, model and year of the toilet is also helpful. This way finding replacement parts is quick and easy and your toilet will be fixed in no time.