I have attached a couple screen captures. Your screens may very a little from mine as I have additional options as an Admn.
First one is to create a new thread (new series of posts for your topic). Click on New Thread Forum Options and then New Thread. This will create your own thread which people can reply with posts.
The second one is to reply to a post in a thread. Click on Thread Options and then New Reply (also an option to also create a New Thread like first example). Using Full Editor gives you some additional options to attach photos and PDF files or include links and YouTube videos.
NOTE: Some forums and threads do not have the option to create a New Thread or Post a Reply.