Start as early as you can!  I started with all three of my kids when they started moving their eyes towards me and responding to my voice.  With my latest baby, I started signing at three months old.  The first sign I did was milk since I was breast feeding her.  I would sign milk and then nurse her.  I signed it before every feeding.   But start teaching your baby signs when you think he or she is ready, even if they are not ready to sign back yet.  Simple signs such as milk, eat, nap.

Babies usually learn to sign before they learn to talk.  Get started as early as you can.  Once your baby can sign his or her needs to you, communicating becomes easier.  Talking to your baby while signing also encourages him or her to talk with their voice as well as their hands.