Welcome to the forum, Tracy!
Babies generally start signing back to their parents at 8-12 months so most programs suggest starting at 7-9 months. I disaree with most of them...LOL I think no child is too young or too old to start! If you start from birth adding a sign here and there, you will be more apt to sign when your baby is ready to sign back.
Babies learn to sign the same way they learn to speak by exposure. The more you talk to your baby the sooner and better she will learn to vocalize. Likewise, the more you sign the more she will sign.
I personally feel that the
Sign With Your Baby complete learning kit is the best deal out there. For about $50 you get a book, DVD or VHS, Quick Reference guide and a bumper sticker.