A posting belongs to a thread and thread belongs to a node (i.e. "forum"). In other words: you have to create at least 1 forum in order to have threads and posts.
A more flat-like structure, like you want it, would require add-ons / custom coding: The above would still be the case, but you could show all threads directly on forum home instead of a single node, for example.
Edit: You can also try the option "Forums default page" (you find it in the admin cp: admin.php?options/groups/basicBoard/). You can set it to "New posts". But also in this case the forums list would be still available by hitting a button on the upper right of the forums/ route.