The discussion is displayed as one post under the other, oldest at the top newest below. This is the easiest way to read the debate. Work your way through the posts to find out what has already been said. 

When you feel the urge to respond select the post you wish to comment on and select the ‘Reply’ button.

A writing box will appear with a set of text tools to help you compose your comment. Once you are happy with your message click the ‘Post’ button to add your point of view to the discussion.