Welcome, Jim!
Just plunge right in - ask, answer like you know what you are talking about ...
When I first started responding in these online umpire forums, I found that the effort required to look up the rules to be sure I was correct was an excellent learning exercise - an enjoyable way of actually reading and studying the books. I say "books" - plural - because I'd recommend strongly that you buy the case book for the organization you call for. It's well worth it.
(After Glen's response, I had to use a few graphics!)