I've also had this same situation with my toilet and it can be very annoying. I researched your question and found a link that may help you with your toilet running. I have provided the link below. I hope this helps!
Shut off the water to just the tiolet ,take off the lid & make the water line inside tank with pencil & in 10 or 15 min check pencil line,if the water level has dropped,then you need to change the flapper ball..can remove & take to store for match.shadester
This issue is all too common and this site alone has plenty of guidelines and links for you to review. Note that the mechanics within the tank are sitting in water - metal pieces may rust out. In one of the Chicago apartments I rented, I removed the toilet tank lid at one point to find that a paper clip was connected the flusher to the chain on the flapper - it worked for awhile:)