@donatelloMay 28.2011 — #Building a forum is rather complicated, what you should do is simply use a free open source forum software that you can install on a Linux Server.
Look into phpBB or Vanilla Forums - both are free and open source.
@LiveCityMay 29.2011 — #I would suggest you to try out websites that offer web interfaces, rather than install-able programs. As such programs are browser-based, you should be able to use them regardless of your operating system.
Feel free to test-drive our CMS, which was tested on Firefox under Mac as well. The link is in my signature.