I'd definitely say to learn (X)HTML first - it's the language the internet is built on. Then CSS adds a bit of flair (although that's more on the design side of things, you can deal with functionality without ever even touching CSS)
Learning C(++) isn't really vital for today's web-world, but it helps get you a good idea of how programming in general works. It's a tad more complex than PHP though, so I'd probably say PHP is best to learn first
ASP is just another language similar to PHP - you can get by fine only knowing PHP although ASP would be handy too, you could come back to that later
AJAX doesn't really have anything to do with PHP, and it isn't really a language, more a "way of doing things" - it's the way Javascript (yep, important number 3!) can communicate seamlessly with a webserver running PHP - but it doesn't need to be PHP, it could be ASP, or C, or Perl, or anything like that.
I'd also recommend looking into a Javascript framework, like MooTools, or jQuery, as it makes all the cross-browser issues that Javascript has a lot less complex, but learn a spot of Javascript first, just to get a feel for what it does. Then apply a Javascript framework to take all the hassle out of it
Hope that's been useful