Student or Learner
What is the expression "Don't get me started!!" supposed to stand for?
It IS slang, is it not? (I've heard it as an exclamation, I'm pretty sure...)
Is its origin known? (geographical and contextual)?
Not a teacher.
You would say "Don't get me started!" if someone started a conversation about something you felt deeply about. For example, let's say that you really like the Lakers (a basketball team), but they've been very disappointing recently:
"So how about them Lakers?" (not correct, but a common conversation starter)
"Oh man, don't get me started."
Or that you really don't like a political viewpoint.
"So what do you think about <political view>?"
"Don't even get me started on that."
The phrase is used to show that you are passionate about the subject, and that you are willing to speak for quite a while about it if they continue. Many times, the person who says this will continue anyway.
I wouldn't really call it slang. It is just a phrase with a specific meaning. It is probably not formal, however; and you would not use it in formal papers (you wouldn't use the first person in formal papers anyway).
I have no idea as to the origins of this phrase.