At the top and bottom of the list of posts on a topic there is a "> Reply" button. That will bring up a similar page to the one you use when posting a new thread.
Or at the bottom of the page after the "> Reply" button is a "Quick Reply" section, you can just type a post into that then click the "Post Quick Reply" button.
Or, if you want to respond to a particular post, you can click on the "> Quote" or "> Quick Reply" buttons that appear underneath each post. The latter just moves the page to the Quick Reply section.
The first of those options opens a new page, like using "> Reply," but it will also include a quoted copy of the full post in the Message box. You are expected to edit this to just the section you wish to quote, then write your response underneath. When editing you just delete the parts of the post you do not need to include, but you must leave the "QUOTE" and "/QUOTE" parts in square brackets alone, these are what will make it stand out as a quote and not part of your message.