Replace the flapper, it's worn out. If you don't believe it, shut the supply off and let it sit overnight, you'll end up with an empty tank in the morning. It's possible that there's something caught on where the flapper seals or the chain is caught underneath, but it's almost always the flapper. The hose that goes into the overflow tube must be above the overflow, or it can act as a siphon. There should be a clip holding it up, and not just stuck down the tube. One other thing it could be, but it doesn't happen too often, is if the seat gets cracked or chipped or loose. Start with the flapper, it'll probably fix it.