Why does Franklin list temperance as the first of his thirteen virtues?

Benjamin Franklin chooses temperance as the first of his thirteen virtues of life because it is this virtue that allows one to develop the self-discipline necessary to attain the other twelve. The cultivation of temperance leads to the development of a cool, clear head, which as Franklin points out is necessary to maintain constant vigilance against bad habits and temptations to overindulge in food or drink.
Franklin believes that if you can conquer your primal urges to eat and drink then you'll have the confidence to make the necessary improvements in other areas of your life, to which the other twelve of his virtues relate. Temperance is undoubtedly the hardest of Franklin's virtues to follow, not least because it requires the sustained application of will-power. All the more reason, then, to deal with it first. That way, adhering to the remaining twelve virtues will be so much easier.