well you have a balance scale...
1. you weigh 6 on one side, and 6 on the other. The pile with the 1 heavier coin is heavier, so you can discard the 6 other coins on the other side of the scale.
2. you now have 6 coins, 1 of which is suspect zero. put 3 coins on either side of the scale. the coins with the heavier coin weigh more, so take them aside, and discard the other 3 coins.
3. you now have 3 coins left. take any 2 and weigh them. if they are the same weight, the scales are balanced and the heavier coin is the one that is not on the scale. if one of the coins is heavier, it will weigh the scale in its favour. either way, you have your answer using only the 3 measurements.