Start with learning proper, standards compliant xhtml and CSS. Once you've got that down, pick either Javascript or PHP to focus on. They are syntactically quite similar, so once you master one the other won't be that hard to pick up. Once you've mastered one, move on to the other.
AJAX is quite simple really. It basically just allows Javascript to interact with server side scripts like PHP. AJAX isn't a language, as all AJAX is coded using Javascript, so once you get a good grip of Javascript it's very easy.
Perl is also a great language, but I find it's a lot harder to setup and use than PHP is. Maybe I've just had bad luck with it, I dunno. It can do anything PHP can do and more, but it's a little more involved.
As far as ASP, I wouldn't bother learning it unless you just want to for the sake of it. It can only be used on Windows, and the majority of websites are run under Linux...so, you'd be using Perl/PHP.
Hope that helps.
Oh, and, make these sites your friends:
W3Schools Online Web Tutorials Web Design Library — One-stop Web Design Resource EchoEcho.Com - Web Hosting - JavaScript Tutorial - HTML tutorial - CSS Tutorial - Flash Tutorial - Online for free!