New job

What is everyone's opinion on telling your employer that you are Autistic?

If you have told them, how did you tell them? And what did they do to try and support you?

If you haven't told them, is there a reason why?

I am starting a new job in just over a weeks time and am trying to prepare. I am thinking I want to tell them so they can support me but I am unsure how.
It would be good to hear what other people have done...