Stand if the dealer is showing a 4, 5 or 6.
Otherwise take a hit.
If the dealer has a 4, 5, or 6, then there is a great chance that the dealer has a stiff hand and will bust. You don't want to risk busting your 12 if there is a good chance of a dealer bust, because then you'll lose instead of winning.
Any other upcard that the dealer has gives her a much smaller chance of busting, and since you have a 12, there are a lot of cards that can improve your hand, so it's worth taking the hit if the dealer doesn't have the 4, 5 or 6 showing.