Toilet won't stop running.
There are dozens of configurations out there, so I can't say without seeing it. But, the float is normally adjustable on the metal rod. It usually involves a little friction clip that you pinch, and the float can slide to a different position on the rod. If your float isn't secured to the rod, there's a good chance that the rod isn't being pulled upward by the float, and that's stopping the fill valve from shutting off.
Pull up on the float while the toilet is filling and let me know what happens.
Then pull up on the metal rod when the toilet is filling and let me know what happens.
If neither of those things stop it, you might take a picture of the tank's inner workings and post it here.