Sound like a leaky flapper to me. Look for a thing in the middle of the tank at the bottom that flaps up for a few seconds after you flush. Chances are it is not getting a good seal. You may need to pick up a new one at the hardware store. I believe that there are 'universal' replacements for cheap. You can also buy a complete guts assembly for about fourteen dollars. The only tools needed are some good pliers and maybe a Screwdriver.