i started signing "milk" to my daughter when she was 3 months old, she understood it by 4-1/2 months! (i'd sign for milk, and she'd either get excited, or look away.) my favorite website is:
Born2Sign, i like the little movies of the signs being done by an adult or little child. i've also ordered some
laminated flash cards on a ring for the most common signs, they have a picture of a child signing on one side and adult on the other, you can make it into a little book by slipping the cards into a "brag book" photo album. babies love looking at baby and kid pictures!