If you want to create a forum, you do not actually need a separate forum software, you can create forums on wordpress site. If you want to create forums on wordpress the following plugins will help you to convert your simple wordpres site into a powerful forum. Some of the popular forum plugins are:
wpforo
Asgaros Forum
bbPress
Discussion Board, etc.
wpforo
Asgaros Forum
bbPress
Discussion Board, etc.