Whatever system you choose doesn't really matter. What is important is that you're consistent and always say the word at the same time as signing.
You're not teaching sign language as such, you're giving baby extra gestures to use. So all babies wave and point - you're just adding to that.
I went to Sing and Sign classes with ds when he was 7 months. He was using about 6 or so signs by 10 months. It's based on Makaton amongst other things.
A sign language for hearing impaired people will be different and harder to master with ickle baby hands, something Makaton based is simple and only needs one word per sentence.
So you would say "Would you like something to eat" but only sign "eat", not would you like.
Sing and Sign have a website and a couple of DVDs.
Makaton signs can be found on the Cbeebies website as part of the Something SPecial mini-site. www.bbc.co.uk/cbeebies
Other methods/classes I've heard good things about are Tinytalk and Signaling (I think)
I would start having a look at the signs yourself over the next few months. Choose a few basics and introduce them at about 7/8 months. If babies are going to get along well with signing (and not all do) then it's usually between 10-14 months that they'll start signing back. They'll drop the signs as they learn to speak.