First, make sure you're on the discussion board page for the book you intend to discuss (navigate to the proper board using the menu at the top of the page).
Second, click on the title of one of the discussion questions.
Third, in the page that opens, post your comments in the box at the bottom. Note that you will be required to provide your name and email address. The former will appear with your post; the latter will not.
A moderator will monitor the boards, so please keep your discussion civil!