For those who need a brief description on the languages, here it is – HTML (Hypertext Markup Language), CSS (Cascading Style Sheets) and XML (eXtensible Markup language) are the three commonest languages used for designing. While the basic language is HTML, CSS is the language used to make the site more attractive. XML is used for programming. Now, XHTML has been formulated which is the reformulation of HTML 4 as per the XML application.

Which Language to Choose?

As per the above description, the three are needed together. Without HTML, CSS is not effective and without CSS, HTML looks like a plain white page with some text. So, rather than focusing on any one, there should be a balance of all.

you didnt describe XM and why its useful.

Well you pretty much answered it yourself, without HTML, CSS is absolutly useless as all it does it describe how the html document should be formatted. So the most basic of things to learn is HTML and CSS as they are the bare basics of web designing these days, if you can master them then take it a step further and look at another language like Javascript, XML or even a backend language like ASP or PHP which take your basic html page with css formatting to another level.