I learned ASL when I was in 4th grade. I befriended several deaf students and wished to be able to better communicate with them. I committed to arriving at school an hour before classes to learn ASL with the Special Education teacher. As a teenager I taught myself more ASL through books. I start with my babies by signing simple words that they will be able to use to 'talk' to me easily.
Children pick up signing easily and quickly.
As they grow they learn more signs. Continue signing the 'right' way and they will sign that way when they are able to. Just as when speaking a language, babies say words similar to a word, you know what he is trying to say even if it is off a bit, and when he has the ability to say it 'right' he will. Don't hold him back by signing 'baby talk'. By being able to communicate their needs, my children do not go through that frustrating time where they know what they want to say, but are unable to say the words.