If you want to learn quickly and build skills that you take forward, I'd recommend looking at ASP before jumping into PHP. PHP is more difficult. ASP is really kind of outdated, but if you learn a bit about it, you'll be able to transition to more difficult concepts that you'll encounter in PHP. I don't recommend using ASP for production, but it is a good learning tool. Keep in mind though that it is very much outdated now. The current model is ASP.NET, which is VERY different.
ASP.NET is something to look at down the road though. Don't try to get into it as a beginner because it's simply far too advanced. It combines all the difficulty of PHP with a more abstract programming model and a huge framework. ASP.NET MVC is really the kind of thing to aim for long term as it follows very good programming methods that are simply nightmarish to do in PHP.
You might want to look at PHP before looking at ASP though, just to get a flavor for it. PHP is more C-ish or more like Perl than ASP. Perl is pretty much the most obtuse web programming language out there, and really only recommended for hard core masochists.
(Well, it's actually quite good and there's a huge amount of Perl code available, but it's simply terse and difficult.)
Do try to find out what XML is though. XHTML is based on XML, so knowing what XML is will help you understand XHTML. You don't need to know how to program XML, but at least know what "well formed" means because that's a key concept that you can't ignore. (HTML is not well formed, but XHTML is.)
Oh, just a cautionary note... Do try to develop good habits early on. There are already far too many bad web programmers out there creating bad web sites and bad user experiences. Along the way you'll discover more and more things like error checking that are core tools that you should be using. Don't get caught up in things that frustrate you though -- just keep going and returning to them as you learn more.
Have fun~! Web development can be a lot of fun and should be enjoyable for you!