If you want to reply directly to the initial discussion then use the "Reply to xxx" box/button immediately under the initial discussion post.
Once a discussion has received replies you can then either reply to the whole discussion or to an individual comment (reply) within the discussion.
To reply to the whole discussion either use the "Reply to xxx" box at the top of the page or use the "Reply to xxx's discussion" button at the bottom of the page. This puts your reply in the main "thread" of the discussion.
There is also a "Start your own discussion" button at the bottom of the page if you feel the discussion you are reading does not apply to your query and you want to ask a new question. This prevents the discussion going "off topic".
You can also reply directly to an individual comment within the thread of a discussion by clicking the appropriate "Reply" button in the individual comment. This puts your reply into a "nested" conversation under the post you replied to and other users can join in this conversation by replying to comments within the nested conversation. Each reply states who the post is from and directed to.