HTML by itself is pretty basic. It is simply a markup language, and as such you can't really do any real processing with it.

Java can be very useful. I personally find it to be a terribly awful language, but it's quite widely used, and it has advantages over other languages because it can interface with the web quite easily.

Flash (the language is actually called ActionScript) can be useful for animations and flashy interfaces, but again, it's not really a language that you'd write programs in that do processing, like you can in Java.

What you might also want to learn is JavaScript - it's a scripting language that is used to do processing on websites. Although it shares it's name with Java, the similarities are only superficial.