I started signing to Sasha when she was just a couple of months old. The signs that are suggested as first signs are milk, eat and more. However, they advise that you choose whatever you would use the most. Eat wasn't a good one for us because we started so young. I use milk, but not consistently enough. The one sign I have used most consistently is finished. (They recommend starting with three signs only.) I use it to ask Sasha if she is finished on the potty. I ask and sign, then answer back and sign. Sasha recognizes the sign now as meaning it is time to get off the potty. She responds by hopping a bit and putting her arms up. I am terribly pleased with this development, but it also urges me to sign more consistently. I need to use finished in more situations and add a couple more words that Daddy and I will both use often. I'm looking forward to adding a sign for potty, but I think it is too soon.
I am open to suggestions of words to sign! Anyone know the sign for teething ring? That might be a good one for us.
By the way, I recommend the DVD set linked above, though I don't recommend television for babies. I do feel better about television for babies if you're watching good programming with them. I also recommend this book that was really my start to baby signing. You can read my review of this book published on Hobo Mama's blog.