If you asked me 3 years ago what language you should learn I would’ve said Ruby. If you asked me 6 years ago, it was PHP.
But where to begin?
Install Firefox and install Firebug. I still think Firebug is the best setup. Webkit is catching up. Watch the introductory video on the Firebug site to see how it works. As my friend and colleague Leevi Graham said “Firebug… still keeping firefox relevant” (source). A new version just arrived.