If your kids watch TV then Signing Time is a great resource. Many of the dvds are available at libraries and some parts of the country still show some episodes on PBS. If the kids don't watch TV but you do I still suggest you get a hold of some to watch because one great thing about them is that in addition to an adult showing you the proper formations of each sign they show several different young children using the sign. This way you see the typical approximations toddlers make and will more easily recognize your child's efforts. Signing Time is also, to my knowledge, the only program that uses all true ASL signs. The other popular programs are all combinations of some ASL, some Signed-English and some "we're making up a few signs for our program because we think the real sign is too difficult for children". One word of caution, the songs on the dvd are cute enough but are of the type that sear themselves into your brain and you find yourself unintentionally humming the tunes while cooking dinner.
The real key to success with sign language with children is consistency. Pick a few signs to begin with, like 'more' and 'eat' and 'drink', and use them every. single. time. As long as you are consistent the child will pick-up the signs quickly. As with spoken language though realize that comprehension precludes usage so he may understand your signs for a few days or weeks before he begins using them. Since you have older children I would teach them the signs and encourage everyone to use them with the baby as this will help make them significant to him. My ds (an only child) used only a couple of signs for a few months but after we attended a free sign language story group one day he suddenly began using dozens of signs he had see me using. Something about seeing more people use them just helped the connection click in his brain.
You can follow a program if that suits your style but there are plenty of free resources available to teach you signs. Libraries always have "The Joy of Signing" and other sign language dictionaries. Online you can see some signs being performed for free
here and
here, or for a small monthly fee
here.
Have fun!