If you have learnt HTML and CSS then you have two main routes open to you from there, the natural split in web development between front-end and back-end. For example, if you like design then I'd suggest javascript and a graphic package like Photoshop (or whatever). The backend would be to look at a language like Ruby/Python/Perl/PHP/C#, and SQL for databases.
You'll need to know a little bit of everything in the end, so have a go with everything just so you're aware of it all, and then you can end up where you're most comfortable (and forge helpful working relationships with those who like bits you dont :)