A leaking toilet can waste 200+ gallons of water per day.  One of the biggest  issues with an increase in water usage is a toilet that keeps running after you flush it or runs by itself without being flushed. If left untreated, this problem can run up your water bill, so you should fix this problem as soon as possible. 

Be sure to check the flapper. If the flapper is leaking, it will keep triggering the fill valve to refill the tank. To see if the flapper is leaking, here is a simple test:

-Put a few drops of food coloring in the tank.
-Let it sit for 25 minutes.
-If the color leaks into the toilet bowl, then there's a leak.

Before you replace it, first check the flapper to see if sediment or debris has built up around it. If not, replace it. This is typically a lot cheaper and easier to replace than the fill valve.