According to the research described in the book, children who sign have higher IQ, had bigger vocabulary, and are less frustrated (and so are the parents). Many of the children talked sooner, have better memory and spacial grasp.
Teaching sign language is easy - just show the sign when you say the word (a few times maybe). In the same way your child learns to wave goodbye - she can say anything usingsign language before her mouth muscles are ready much later.
ASL (american sign language) is what the researchers used, but any gestures can work - especially ones that the baby herself invents.
Beginning signing can be done as early as you want and can benefit until the age of 2.5. You should expect your child to sign back and the time you expect her to wave, and you can gently guide her hands to speed the process.
In order to learn it yourself - there are many examples in the book - but you can get an app demonstrating the signs (you can start with just a few at first), or even watch a youtube video like this one.