If it is first time, then prepare a short introduction for yourself. Most likely you will be asked to say something about yourself - 2-3 sentences, including who your favorite author is.

Second - you are not obligated to actively participate. First time in a group, it is always good to just observe and feel the vibe.

What happens in a book club - The organizer gives people a list of books to pick from, the group votes on it, and the designated book is chosen. Then you are supposed to read it and come for the next meeting where it will be discussed. Different book clubs may have variations of this procedure with differences.