PHP and JavaScript would be great choices ... knowing CSS and JavaScript especially can open a lot of doors for dHTML and eventually AJAX, but AJAX does need a lot of server side coding as well. Web languages are always a good starting point, since they're so widely used and you can get into them at any level of difficulty and gradually build your knowledge.

if you're up for more in depth programming, learning Java / JSP would be a nice choice, since then you wouldn't be limited to web apps. Armed with a nice IDE like Eclipse, Java can be easy (almost fun) to write once you get wrapped around the OO mindset... Design is a lot more important with it (and any OO language) though, since there can be so many little things going on.

C is pretty hardcore, but a great language when you know it; probably not the best choice for a beginner.

Perl is another really nice option... It's really powerful (especially on Linux) and its loose syntax makes it quick and easy to write.

Python is definitely worth looking at too... gaining popularity very quickly. Next language on my own list to learn.