What programming knowledge do I need to create web2.0 apps?

You don’t need Java necessarily which would be very advanced. HTML and CSS aren’t programming languages. To call something an application you need some kind of business logic. Javascript is an easy way to get started. You could also try PHP or Perl.

PHP is a pretty good bet, it’s free so there’s a lot of help for it. Facebook for example runs on PHP.

Yep, you’ll need a server-side language for the business logic (php, asp, whatever). What does set most web2.0 applications apart from more traditional websites is a heavy use of javascript (for your eyecandy and ajax responses). Just look at fluther for many examples (great question, crafting a response, etc).

There are a number of server-side languages you can learn; you’ll need to learn at least one. Perl, PHP, Java, Ruby, Visual Basic, and C# are the big ones on the server-side. I disagree with @mattbrowne in that I don’t think that Java is necessarily more advanced than other languages. Different languages have different learning curves and power/performance sweet spots, but once you get past trivial problems, the problem you’re trying to solve is going to dictate a lot more of the complexity than the language you’re solving it in.

You’ll also need to learn the client-side language. Javascript is found across the board on all browsers, and no other language is, so you need to learn that to an acceptable level of fluency.

On top of that, you’ll need to know HTML and CSS thoroughly—and it’s not that they’re terribly complex, but that you have to be aware of where various browsers (I’m looking at you, MSIE) have bugs and inconsistencies in their implementations.

@cwilbur – Yes, Java isn’t necessarily more advanced than other languages, but it’s been my experience that many lateral hires coming from office macros, HTML, CSS and so forth find it easier to start with server-side script languages. There are people who have some trouble grasping the concepts of classes and objects or that of a garbage collector. During one job interview I had an applicant who looked at me with the question written on his face, ‘do you want me to vacuum the office floor?’ We never got to the point of discussing enterprise java beans. Coffee anyone?