If you become serious about web development (front and back-end), you'll want to focus on HTML 5 and PHP (as previously mentioned), as well as some JavaScript (just enough to do what you want your applications to do). PHP's syntax is neat.
For testing out PHP (or other web programming languages, with Perl and Python remaining popular), it may be convenient for you to set up a local server. Consult the XMPP website for the most convenient solution. It's a configured server setup in a box.
NB that I can vouch for immortal's recommendation of W3Schools.