The most common cause of running toilets is an old flapper that needs to be replaced. When flappers get old, the seal is bad and causes water to leak to the toilet.
To replace the flapper, remove the old one. Replace it with a new one that you can get from any hardware store. To remove the old one, detach the chain and take off the flapper. To attach the new one, connect the chain and reattach the flapper from the sides to the pins.
Tip: Make sure you drain the water from the tank before replacing the flapper. Shut off the water supply to the toilet behind the toilet. Flush the toilet to drain the water.
Tip: You may need to shorten the length of the new chain. This is a problem if the chain can get caught underneath the flapper.