There are some stages most babies follow when learning to sign. If you haven't seen it, experienced it, it can be hard to comprehend just what those stages look like. Here are some video clips that may help.
Step one - Notice that people are signing
Step 2 - Respond to the signing
Step 3 - Beging moving hands in purposeful ways
Step 4 - Use single signs to communicate
Step 5 - String signs together to communicate
Step 6 - Begin to say the words being signed
Step 7 - Speak and sign a lot!