There are a number or possible causes for this problem. It could be that the flapper isn't seating properly. The flapper can accumulate a thin layer of scale or a slippery film over time which can cause the water seal to fail. Cleaning or replacing the flapper will correct this problem. Another cause can be a kinked or tangled lift chain which is easily fixed as well. If neither of these is the problem it could be a worn out flush valve. There are a number of different kinds of assemblies available at any hardware store, plumbing supply company and most discount centers and installation is fairly straightforward.