You can start programming web pages very quickly - what you should focus on depends on what you want to do.
No matter what, having familiarity with xhtml is important, so that is something you can start with immediately in any case.
Along with xhtml, you will want to learn about cascading stylesheets (css). These are both fundamental to web development.
However, after that there your path will depend on what you are interested in doing. In its most general sense "building websites" covers a a wide variety of skills and knowledge. Website development touches in all these areas (and more): Graphics, layout, art, server setup and administration, security, programming (both client side and server side), general computer knowledge, writing, accessibility, "web centric" things such as html, css, Web 2.0, blogs/wikis/blikis/forums/video/podcasting, dynamic web sites, Ajax, web services, and so on... and etc. It goes on and on. For a hobbiest, you have to get some skill in a number of these areas - but you can start with the basics and follow along into any area that you find interesting.
The essential thing to understand is that you can put together simple websites with very little knowledge, and learn the things you need to know to do anything you want to do.
How to use a forum to help solve problems
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