Basically a cms is a combination of various things that, combined allow you to build a system such as a forum platform or blog engine.

I am busy at the moment building a blog engine and this was my learning curve:

1. HTML (XHTML) – certainly to the point of an advanced user.
2. CSS – same. I spend a lot of time learning CSS right from the pages of the specs.
3. JavaScript – honestly, I am not an advanced user, but do love to play with various features. You want to get a handle on Ajax and most imp form handling. Forms (like the ones we are communicating with now) are the bread and butter of the interactive web.
4. This one is obvious – a server side scripting language. This is the fundamentals of a cms. If you are even asking about cms then you probably are already studying something like PHP. Could be Python or Perl or various others, but PHP is so simple to learn and implement.
5. You want to learn both sql (mysql and sqlite are good to start with).
6. xml and most imp, RSS.

That would be enough to build a good blog engine. Never expect to learn it all from tutorials. spend a lot of time building, break your problems down to small parts and ask questions or search online for those specifics and you will learn much faster.

I hope that was the sort of answer you were looking for.
References :
http://www.w3.org/TR/html5/
http://www.w3.org/TR/css3-selectors/
http://www.w3schools.com/js/default.asp
http://www.tuxradar.com/practicalphp
http://www.w3schools.com/rss/
http://www.xul.fr/en-xml-rss.html