1.) Most common cause is the flapper is not seating or is worn and needs to be replaced.
2.) The chain could be catching on something causing the flapper not to seal
3.) The trip lever may be loose causing different chain lengths. This will cause the chain to catch and not let the flapper seal
4.) Replace the flapper. If that doesn’t fix it then the next step is to replace the tank insides. If the fill valve is a few years old it is a good idea to change it with the flapper
5.) Before changing the flapper make sure you turn off the water at the supply line behind the toilet