First take time to review the site by surfing around to discover how it is organized. You may read site content without registering, but you must SIgn Up (free) prior to creating or replying to a Topic.
To create a new Topic, select the category that seems most appropriate for your question or comment. Navigate to that category. Then click the + New Topic button toward the top right of your screen. This will display the text editor whereby you can type your post.
To reply to an existing post, either click the Reply button at the immediate bottom of a post to reply to that specific person who wrote that post, or click the Reply button at the very bottom of the entire thread of posts in order to reply to the Topic as a whole.