Use "Yummy!" to describe food that tastes good.
"Yummy" has a feminine and somewhat childish sound to it, though. You wouldn't expect to hear a big, muscular straight man describe his food as "yummy" to his buddies. More often, adults say "yummy" to children or women say "yummy" to other women.
You can use "yummy" in two different ways:
Oh, and women use "yummy" in one other way: to describe a man who's really attractive and sexy:
He is so yummy.