First you want to do as RedTurtle says, and do it successfully. You can investigate Joomla and see if it's right for you at joomla dot org. Here's a friend's Joomla powered site, for example:
http://smokeyandthebandwidth.com/
If you want Joomla you just download it from the joomla site, then load it on your server as the main site. What you do with it from there is up to you.
Also, look at this:
https://vborg.vbsupport.ru/showthread.php?t=228112