If the water level inside the tank is right at the top of the large overflow tube, then the problem is a leaky fill valve not shutting off.
If the tank water level is normal, then check to see if the small black refill tube is stuffed down inside the larger overflow tube. If so, it is siphoning water. You can get a bracket to position it ABOVE the overflow. You can even cobble a bracket up from a paper clip.
If none of the above, then it is simply a leaky flapper. Assuming this is a Wellworth, or actually no matter what model, it is a KOHLER, so the "generic" flappers at the hardware store are not right. Some of the common Kohler flappers are available at HD or Lowes. Otherwise, take it with you to a plumging store for a proper replacement.