It means "This is a topic I feel very strongly about. If I get started on it, I am going to rant and rave about it and go on and on and keep talking about it until you and everybody else are bored." It's a way of saying "Let's just change the subject, I don't want to get involved in an intense debate about this now."

:)) you usually use this when you have a lot to say (usually bad things, rants) about something. for example, let's say you've been to a restaurant and it was awful, then you tell your friend about it: "the chicken was so dry, the X was so Y, and don't even get me started on the bread! / and don't even get me started on how badly the place smelled! / and don't even get me started about the service!