There are normally three solutions to this problem and one of them should be the answer to your toilet automatically filling. The most common problem is that the flapper needs to be replaced. That is the little black ball looking thing at the bottom of the toilet normally in the middle. It is attached using either ears or by slipping over the overflow tube. Remove this piece by lifting it off or removing the ears from the overflow assembly. Be sure to bring this to your local retailer, there are many different types. Another problem may be that the flush valve is nicked. If you have replaced the flapper and it is still running then take the flapper off and run your finger along the plastic or metal rim that the flapper made contact with. This piece could very well have a small nick in it and replacing it is necessary. The last reason your toilet would have these problems is because water is going over the overflow tube and not shutting off low enough in the tank. If you can see that the water is above the marked water line you may want to lower it back down. To do so carefully bend the arm that protrudes out of the fill valve to the black float ball. Bend this rod down. If you have a fluidmaster (has no rod or ball) then you simply squeeze the clip in the metal wire connecting the fill valve to the float and lower the float.