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Click on - Control Panel >> Global Configuration >> Server Tab >> Select Entire Site from Force SSL >> Click on Save tab
yeah, that really easy, but don't forget to setup SSL on your server 1st
Let's bet which dot-name will collapse first
Oh, and if your using Akeeba's AdminTools and have problems with external links, make sure you go to "SEO and Link Tools" and disable "Convert all links to HTTPS when site is accessed over SSL". This setting adds "https://" to every external link found on the site, including the forum's. Caused many people many headaches in the past.
@tommy: Pre Requisites for enabling SSL in Joomla is here