For All The Marbles

Weighing Marbles Can Be Both Fun AND Sexy

You have twelve marbles, labeled 1 - 12. One of them weights slightly more OR slightly less, (you're not sure which), and the difference is imperceptible to you. You have a balance, (one of those antiquated devices that blind women in front of courthouses like holding). Using this tool, what is the minimum number of weighings that you can use to ALWAYS find:
a. which of the twelve is different
b. whether the different marble weights more or less than his sister marbles

Answering just a number of weighings is not enough for full credit; you need to explain why. Good luck!