Food is one of the most common first signs. Make the sign whenever you offer food to your baby, when she eats, and just after she has finished. Your baby will be highly motivated to learn this sign, and will quickly learn that using the sign enables her to get food when she is hungry. Hungry fussing, be gone! As your baby gets more advanced, start introducing the names of specific foods, so along with the general word food, your baby starts using more specific words like apple, or orange.
Eating is one of our favorite signing opportunities. We sign food, we sign more, and we sign all done as part of our meal repertoire.