Milk

Signing: The milk sign is a lot like milking a cow (or goat), but without the vertical motion – you are just squeezing the udder. You take both hands, make them into a fist, relax, and repeat.
You will notice most babies have trouble moving all fingers together at uniform speeds, but any kind of repeated squeezing and relaxing of the hand is likely milk.

Usage: Milk would be one of the first signs that I would teach baby, because it corresponds to something they find highly desirable, and so they are highly motivated. Also, teaching the milk sign helps reduce frustration where baby wants to feed but cannot communicate it effectively in any other way than being cranky. Make make the milk sign before you feed baby. If baby takes a break during feeding, then wants to feed some more, that is a good opportunity to do the sign again. And you can do the sign one final time once feeding stops.
At first, whenever baby makes the sign – or is close, fuss over baby and try and immediately give her milk.