A good order for learning a new piece of software is
learn how to install Joomla
Backup your joomla website database, and site files (pretty easy with FTP, and joomla pack)
adding content (use JCE editor: http://www.joomlacontenteditor.net/ and use FTP to upload images to the image directory on your joomla site )
changing and editing templates, the navigation bar templates (try to avoid editing the php, html scripts if you do not know what you are doing, or atleast make a backup)
adding modules, plugins, and components (You might have to enable the modules, plugin. in the admin panel after you install the module, or plugin)
It took about 2 weeks for most of the students in my class to figure it out. But, the sites that we made were very basic compared to other Joomla sites on the internet.
You should be able to figure out faster since you know how to edit your forum. Some of Joomla features like the navigation bar is similar to phpBB, the default WYSIWYG editor for making content is somewhat similar to Mxproject free webhosting builder: http://www.mxproject.com/freehost/
