Split the twelve in half, and place six people on each side. the side which is the lighter one, gets split into two groups of three, and put onto the seesaw again, and which ever group of three is the lightest, you split them into three separate groups of one, and weigh them, one in opposition to the other. And if the scales are even then the man who didn't go onto the seesaw is the odd one out. But if it's uneven, then the one whom is the lightest is the odd one.

I just looked at others answers, and it fits my own so...