After reading a comment you might want to reply to it, you can do so by clicking the "reply" link at the bottom-left of the comment.
On the page that appears, fill in the "Comment" field (it is required). You can also optionally supply a new subject in the "Subject" field. You cannot attach files to a reply. You can, however, add click-able links to your reply. To do so, select the text that has to be turned into a link and click the second button from the left on the button bar above the "Comment" field. Fill in the "Link URL" and the "Link title" fields and click "OK".
When finished, click "Preview" on the bottom of the page to have a preview of your reply. When satisfied with the outcome, click "Save" to actually post your reply on the forum.