The illustration shown here will give you the proper terminology for each of the parts you may come across. The photo below shows the newer and more effective flapper rather than a drop ball.

Step 1: First check the guide rod or chain on the tank stopper. If the rod is bent or the chain links are twisted, just straighten them.

Step 2: Next check the float mechanism. If lifting the ball up stops the water from running, try to bend the float arm down to get the right buoyancy. If you notice that the ball has water in it, it needs to be replaced. Replace it by unscrewing it from the arm and putting a new one in its place.

Step 3: If the toilet is still running, the valve seat and stopper may have corrosion or build-up preventing the stopper from closing. Lift the stopper up and check for any objects. Gently scour the seat and the rim. If there is a great deal of damage, replace the stopper and valve seat.

Step 4: The flush valve assembly may have to be replaced if the toilet is still running. Take the old parts with you when purchasing new inside gaskets and assemble to ensure a perfect match. If the shaft of the assembly is cracked, the whole shaft and assembly will need to be replaced. Again take the flush valve assembly with you to get a perfect match.