I recommend starting out with four most common signs (milk, more, all done, and eat) and being consistent. Just sign whenever you say the word to your baby, and soon you will notice your child’s excited reaction when you open and close your hand for milk.
Some babies start signing back as early as 6 months and some may not sign until closer to a year old, especially if they are early rollers/crawlers/walkers. If your baby is breastfed, he/she may use the “milk” sign for mom, too. As your baby starts to sign back to you, feel free to add new signs that are pertinent to your family.
Signing Time also has a great online dictionary with 100 signs you can use with your child. For any signs not included in the dictionary, I highly recommend Lifeprint.com!