I'm trying out the Joomla demo and I wanted to add a forum to the site but I'm not sure how to add one.
You can build forum inside Joomla by installing Forum Component, you can install any of the forums available in extension Directory: http://extensions.joomla.org/extensions/communication/forum . I suggest you to install Kunena Forum as it is open source and has lots of extensions.
can try to use 4rum component like Kunena can check demo here : http://joomquery.com/en/home/communication.html and if you family with phpBB3 try to use it widht bridge sê demo here: http://tinbaohiem.com/baohiem goodluck