First, of course, learn XHTML and CSS. Any web developer should know these things, even if (s)he expects to spend all development time working with a server-side language, because one should understand the interface design limitations when developing the back end to a web application.
If you're just looking to learn technologies on your own, I'd recommend learning PHP next as a way of easing into it. Skip Javascript unless and until you need it for something specific. When in doubt, don't use Javascript. Among other things, this approach will help ensure that you learn how to design a site so that even when Javascript is used, it degrades gracefully.
Once learning some PHP and getting familiar with it enough to kinda get a handle on how web development is done at the low end, move on to Perl, Python, or Ruby. If you're looking to work solely on a Windows platform and are developing for end users that are 100% Internet Explorer users, ASP.NET is a definite option. On the other hand, if you intend to do cross-platform development or general-purpose web development for users who will potentially be using Firefox or other browsers, ASP.NET probably isn't the best option. In fact, if you're doing cross-platform development, it is DEFINITELY not the best option.
If you're already a programmer, it might make sense to skip the PHP step and go straight to Perl, Python, or Ruby. Of these, Perl is the most ubiquitous, and thus probably the most important unless you have specific needs that suggest Python or Ruby as the best first step. On the other hand, the only language more common on the web for web development than Perl is PHP, so keep in mind that despite PHP's deficiencies as a programming language and regardless of your programming experience, PHP may be a best choice for a first language anyway.
Don't start with Java unless you already know Java and know you'll be working pretty much exclusively with Java. Java for web development is something to learn if/when you are going to do some Java-based web development, and not before. The platform and installed system requirements are impressively difficult to maintain in anything less than an enterprise situation, the appropriateness of Java for small web development projects leaves something to be desired (to put it mildly), and stuff like J2EE/JSP constitutes an exceedingly painful system to use in comparison with other web development technologies. In short, if you don't already know that Java is what you have to use, don't bother starting with Java for web development (though you may want to end up there eventually, depending on your career goals).
In summary:
I recommend starting with just learning XHTML/CSS pretty well, then learn PHP, then move up to Perl, then start pursuing whatever catches your fancy -- and fit Javascript in there just whenever it suits your needs and learn web development with Java only if you have to. ASP.NET, VBScript, VB.NET, and C#.NET are appropriate only for single-platform Windows development with only Internet Explorer being used by all your clients -- and some of those (such as VBScript for the client) shouldn't be used even with Windows servers and IE clients unless you're specifically aware of what configuration is on the clients because your IT department controls all clients that will access your site (which limits the usefulness of VBScript on the client side to certain intranet sites).
If you want to get started with PHP and don't know where to begin, there are some things you should know:
1. There's a lot of bad PHP code out there. Don't assume that because something was done in PHP a given way that this is the way it [b]should[/b] be done. Be careful when looking at example PHP code.
2. There are many, many free online resources for PHP out there. Make use of that.
3. You'll eventually want to get a book or two on the subject of web development with PHP. When in doubt, try to aim for stuff that deals with database integrated development, and if all else fails you can generally trust the O'Reilly titles on just about any programming subject. If at all possible, get involved in the open source development community enough to get a firm grasp of what constitutes good and bad code and what constitutes good and bad books on the subject before spending any money. Surprisingly, the best community for learning to recognize good resources for PHP might just be the Perl community -- check out [url=http://www.perlmonks.org][b]PerlMonks[/b][/url] in particular.
If you have the right background knowledge to make use of it, there's a [url=http://arc.apotheon.org/cheats/php.php][b]PHP tutorial[/b][/url] I wrote recently that might prove useful to you in getting started if PHP is where you want to go first for a back-end web development technology to learn. It'll give you the basic knowledge necessary to start making easily-maintained, small websites that use PHP and flat-file data storage. You do need to know some basics of general programming concepts and both XHTML and CSS to be able to really make use of the tutorial, but you don't by any stretch need to be an expert with any of it.
Good luck with whatever you do.