On the first measure you put 4 coin in a plate of the scale and other 4 in the other plate. If one of the plates is heavier then the fake coin is one of this. If not, the heavier coin is in one of the other 4 not measured coins.

Then you put 2 of the coins that may be fake in a plate and the other 2 in the other. The heavier plate has the fake coin.
Finally you put each of the coins in the plate and the heavier one is the fake.