The most common cause of a toilet running especially if the toilet has just suddenly started running is the toilet tank flapper going bad.
The toilet tank flapper is made of rubber and hooked to either a metal or plastic chain and when the toilet flapper starts getting hard overtime it won't seal properly and when the flapper doesn't seal properly it allows the water in the toilet tank to slowly seep out into the bowl and then causes the toilet to run again to fill the tank back up.
First check to make sure the chain isn't kinked.
If the chain isn't kinked then the toilet tank flapper should be changed out. All you do is unhook the toilet flapper and then take it into the hardware store and get an exact replacement.
Then just slip the new flapper on and the toilet should stop running.
If not then your toilet fill valve may need replaced or adjusted.
I replace my toilet tank flapper every 5 years because I find that they start going bad after 5 years so I replace it just to keep the toilet from running and causing my water bill to go up.