To respond to a post that already appears in a blog, move to the bottom of the entry that interests you and click on the word "Comments." There, you will be prompted to add and post your comment.
If you would like to begin discussion on a new topic, add a new post (as opposed to commenting on a previous post). To create a new post, click on "New Post" (found on the right-hand side of the blue strip along the top of the page), provide a title for your post, add your comments, and click on "Publish Post."
Note: If you do not see the "Post" and "Comments" options, that is because the blog administrator has blocked access to these options.