Select the board that you want to post a topic in. For instance For Sale. Once you are in the For Sale section, on your right hand side you will see 4 buttons. The 3rd button is "New Topic" Click on this button. You will be directed to a new window that allows you to select a "subject" This is what others will see before entering the thread. Below that is a series of formatting options for the message, such as adding images, bold, italics, link etc. Just below that is the "message" window which is where the text of the posting will be placed. You can then preview your post to see if it looks the way you want. Once you get it the way you want it click post. You can always edit it later if you want to make changes.
This works for all of the boards, except Club Events. Club Events can be posted in but no new topics by members.