To post (or start) a new thread in a forum you need to click on the Post New Thread button that is located at the top and bottom of the forum on the left hand side. You need to remember to have a title for the new thread.
To reply to post on a thread you just need to click the Reply to Thread button located on the top of and at the bottom of the thread on the left hand side. This will take you to the Quick Reply area where you can reply to a post. You can click on the Go Advanced button and it will take you to another screen with more posting options.