Some years ago my mom gave me a video series that teaches sign language for babies.  I learned a lot of signs that way, but how do you learn a new sign on demand?  You use the dictionary.  There is a wonderful online video dictionary that will teach you countless signs. You can look for the word you want to sign and watch a video clip of someone demonstrating that sign.  You can watch it over and over until you are sure that you are doing it right.

Click on the link and spend some time exploring their website.  There is so much to learn.