Joomla is one of the most popluar Content Management Systems on the Web, not to mention free and Open Source. However If you want to turn your Joomla site into a Blog or otherwise you want to have comments funtionality in your website there is no built-in Funtion or component. I run a blog on Joomla too and first thing i installed was Jomcomment a Commercial Component, which is really very professionally designed component and I love it however it cost a bit $25 or so. However If you want to have Free solution then You can integrate comments into your Joomla site using Intense Debate or even some site use Disqus both of them are free and very helpful too. Hope this would help integrate comments into your Joomla powered site :}