Do you hear your toilet coming on and off when no one is near it, chances are you have a worn flush flapper, which allows water to slowly seep out until the float opens to refill the tank.
If your toilet is just continually running, it could be a worn or stuck flapper or your fill level float is stuck and allowing the valve to stay open.
Either of these issue can waste a lot of water and a lot of money.
They are both less than a $10 fix from the local hardware store, or about $100 to call your local handyman.
Here is a great video showing you how to fix your toilet yourself.