This is usually caused by the flapper valve leaking into the bowl. Unless you have a non-standard toilet, replacing the flapper valve is about a 5-minute task most anyone can do (once you get a replacement). FIrst, though, make sure that the chain is not too long and getting caught underneath it. If you feel the thing, if it is soft and slimy, the rubber is shot, and it needs to be replaced. While you have you hand in there, feel the seat where the flapper valve rests - it should be smooth all around it. I could have a little gunk on it, causing a slow leak as well.