There is a clog in the line. A plunger usually solves this problem, but you have to use a toilet plunger. Your local hardware store will give you the right tool ($13). If the plunger doesn't work, you need a toilet auger (also called a closet auger), which you can buy for less than $20.
Shut the water off flush toilet and there should be two bolts that you can see at the under side of that tank. You need to remove them bolts and lift off the tank and install a new seal. Sometimes i mean sometimes you can tighten them a little and they will stop leaking. If it has been for awhile id just replace it.