I don’t think there is a current WYSIWIG editor plug in that is working. @spirobel was working on one for while but I don’t think it is currently functional:
Tags usage will depend on a number of factors such as forum purpose, user needs, content management, subject variety, etc. I don’t have them enabled on my hosted forum because they don’t really suit that particular forum’s purpose or user base. You can always enable and configure them later when you have your forum setup and running with content you need to organize. These might help:
The styleguide is simply a handy reference for making custom CSS related changes to elements on your forum or understanding what customizable variables and elements exist.
If you are looking for programming tips in Discourse, you can start here