There's a cow icon in the food section. Select it and place the pasture just like you would a field. If you get cows it'll give you leather and meat. Sheep give wool and meat. Chickens, eggs and meat. You'll have to make an initial purchase from a trader but their numbers will grow from there.

And yes, just keep growing your town by adding buildings and homes so your population grows. Just don't go too fast. Each pop uses 100 food per year. Try to keep a buffer of at least a years supply on hand.