At the topic view screen users can see all of the posts that are in the current topic (as long as they are not being moderated in a moderated forum). Users can also use this screen to subscribe to the topic and receive a notification if any replies are made.
If the user has sufficient permission, and the forum is open for posting, there will be a 'New Topic' link for the user to take them to the posting screen. Adding a new post is simply done via the form at the bottom of the page. More information on posting/editing posts is found by following the links below:
Adding a New Post
Adding a New Topic
Editing a Post