I don't know where you read about XHTML Transitional, but it was outdated. The standard now is 'XHTML 1.0 Strict'. Transitional should only be used to accomodate older browsers, but it's better to use Strict. You certainly shouldn't be declaring them as HTML if they're written in XHTML. The languages are almost identical, but there are some fundamental differences in syntax. HTML was designed to be used in making a self-contained website, whereas XHTML was designed to be combined with a CSS stylesheet.
If you go to any decent website and look at the source code, the first line should be the following:
or for HTML:
This is called a "Document Type Declaration" (DTD), often referred to simply as a "doctype" or "doctype dec". It is very important to include a DTD in production websites. If there is no DTD, then certain browsers (ehm MSIE) enter something called "quirks mode". This basically means that the browser assumes the website was written by a "n00b" and tries to compensate by changing bits of your site. Therefore a very nice a well-formatted XHTML/CSS document might not look right.
I always recommend beginners to read through the tutorial on http://htmldog.com/ . It's very well-written and sets you off on the right foot of conforming to W3C standards!
If you want to have your website checked to see if it conforms, go to http://validator.w3.org .
I think it's very wise of you to learn XHTML, CSS and JavaScript as your first step. HTML is kind of irrelavent, and XML is only useful if you have major data-driven websites such as a directory of some kind. Say if you had a business and wanted to create an identical page for every product you offer with a photo, part # and technical information. Instead of creating 200 individual webpages, you'd place all the information in an XML document and call it with JavaScript or PHP.
XML documents are identified by this DTD:
version="version_number"
encoding="encoding_declaration"
standalone="standalone_status" ?>
It's important to note though that a user would never directly view an XML page. The XML files contain information which is arranged/put in order by an XHTML document, which in turn is stylized by a CSS stylesheet!
I like to analogize it to a recipe:
XML = The Ingredients
XHTML = The procedure
CSS = The icing
On most simple websites the XML and XHTML will be in one place. Personally I prefer to use PHP and SQL for my data management, though.
Now getting back to the subject of Dreamweaver. I think it's an excellent program, but one should not attempt to use it without a firm knowledge of their markup and scripting languages. The nice thing about Dreamweaver is that it allows you to instantly visualize your code as well as giving you tips as you type on proper syntax, etc. It also allows you to drag thing into the position you want rather than playing around with the CSS for hours to find the ideal numer of pixels to offset a textbox. :)
Good luck!