What do I learn next?

Javascript.

It really depends on what your interested in and what type of career your looking to get into. Having learned HTML and CSS, i'd assume it's a web related role, therefore the next step you should take is learning JavaScript & jQuery. I think every web professional should have a knowledge of JavaScript and learning it will only help you learn other languages, like PHP for example. Once you've took the time to learn a language, the basic concepts of programming are the same in other languages too, so it's easier to pick them up. My advice is just to follow one of the learning adventures, like the one for becoming a web designer as it covers most of the areas that you would need to know. Also don't be afraid to try out an area of development that you've never tried before and don't think you'd be interested in. You never know, you might love it and it may become your career one day.