Before learning actual programming languages you'll want to get familiar with the basics of the web. Web pages are structured with HTML and styled with CSS, so you should quickly learn how they work. If you were editing pages before using a WYSIWYG tool like Expression Web, its time to start editing the actual HTML and CSS code. You can either do this in a simple program like Notepad++, or within Expression Web from the "code" view so you can still use some of its tools. To start learning HTML, you can go through a tutorial, such as one of the resources here from Mozilla. W3Schools often comes up on Google searches, and they have some useful interactive resources, but beware that they may contain a few inaccuracies. To get an idea of how HTML and CSS is used, you can view the source of any webpage you visit by clicking on "view source" in your browser. You can also play around with the HTML and CSS from within the Chrome developer tools or with Firebug on Firefox. This will let see how many different websites use CSS styles.
HTML on modern websites are not created by hand or by an HTML editor, but by web applications, such as a CMS. Web apps are written in programming languages, such as PHP or Ruby. As discussed, many web apps are written in PHP. So if you want to develop new components for your Wordpress blog or wiki, you need to learn PHP. W3schools has a fair amount of info on PHP, and I don't think it has too many errors. WebMonkey looks like it has a good tutorial for beginners. There aren't many new interactive tutorials on PHP, but there are many books you can purchase.
Stay tuned for the next post on Ruby and Ruby and Rails, the most popular modern framework for creating websites.