What should I learn after HTML?

I’ve been interested in programming and have taken on the task of learning HTML. I’m still in the process…and loving it. I’d basically like to get into web design but have MUCH to learn. What’s next? XHTML, PHP, CSS, something else entirely? What’s my sequence and what’s truly necessary or not? I don’t want to spend time learning things that are outdated or that don’t give me a stepping stone to the next level.