.html is obviously the easiest (and required to do the rest anyway)
.asp is probably the easiest of all the server side scripting languages, however also the least powerful - for many advanced functions you require seperate components to be installed.
.php is the one i would recommend you do if you are just starting a fresh. Comes with many libraries which can allow you to do pretty much anything you need. (also doesn't require a windows server to operate, as true ASP does)
the basics of sql are the same across the board, however mySQL doesn't yet support all the functionality of SQL Server, so code is not always interchangeable.
XML isn't a programming language of sorts but more a way of data exchange - you can therefore use it within your desired language (asp, php, etc etc)
If you are thinking of a career move, Java would be a good language as it is highly sort after (well it is here anyway) and the schematics of it will allow you to pick up C++ easily (or easier than if you only did scripting languages) - however personally I hate it and it provides no fun for me at all....
Recommended course of action - html (and CSS, can be very useful) followed by php + mysql. You'll then be able to make some pretty cool sites from base, and then decide if you want to take anything any further.