If you want to start a new "Thread", which is a brand new discussion on a boating-related topic that you choose, just select the appropriate forum by scrolling through the "Discussion Board" listing (click on it at the top of the page). Then click on "New Thread" in the bar at the top.
If you want to post in an existing thread, such as what you see listed in the "Home Page Portal", look at the bottom left, below the last post, and click on "Post Reply". Or, if you want to address something in another post, you can click on "Quote" within that post, and type your response to it.
You'll get the hang of it, don't worry.