Hi S.,
I taught my daughter to sign when she was about 8-9 months, and it was great! She learned to communicate so she didn't have to cry for things, and as she got older she learned to say the words that went with the signs and eventually dropped the signs. Now she's 3 and has an enormous vocabulary.
The basics that I taught my daughter were "all done," "eat," "milk," and "more" first, then later "please," "thank you," "no," and "yes.
By the time she was 18 months, she knew about 25 signs and could say many of the words she was signing. My son is 11 months, and so far he knows "all done," "milk," "eat," and "no."
Incidentally, my daughter has continued to learn sign language as a second language. We bought the DVD series "Signing Time" by Rachel Coleman. It features Alex and Leah, two kids who help teach sign. Anyway, my daughter absolutely loves it! She talks about Alex and Leah all the time! If you just wanted the basic signs, the very first DVD would be great. (I got them from amazon.com)
All this to say that it has been a great tool in our house, and I highly recommend it! :) Good luck and have fun!