"How should I begin talking to my 12 year old autistic son (high functioning) about puberty?"
Talking about sexuality with an HFA child needs to be straight forward. Autistic individuals do not pick up on social cues, therefore when talking about sexuality it is important to use concrete terms. Use real terms to describe what you are talking about.
Expect that your child will be a sexual being, and understand that with a diagnosis of autism often comes an inability to control impulse behaviors. It is important to be proactive when preparing yourself and your child for puberty.
Teach him that it is okay to be a sexual being, but this is also a private time. Teach him about good touch versus bad touch so that he is not vulnerable. Let him know that you are comfortable (and work at it if you are not) with this type of conversation so that he can be comfortable too.
Sometimes it's difficult to accept this reality (i.e., that they are sexual beings) in our children, especially when they have a developmental challenge. Nonetheless, they need to understand their right to express their sexuality in appropriate ways, but they also need to understand the important of privacy. They need to understand that sexuality, while a social behavior, is constrained by social rules, and they need skills to enable them to behave acceptably in open society.