Seeing how a developer handles a specific problem, how he/she then tries to break it, and then fixes it, is good way to learn how to program in PHP. Moreover, thanks to WordPress's built-in fail-safes—breaking a plugin will, most likely, not break your website.

WordPress's plugin API at http://codex.Wordpress.org/Plugin_API contains all of the WordPress-specific tags, functions, and insights in to making and modifying plugins.

As mentioned before, each plugin is written in PHP, which means that a plugin has access to every PHP function that exists—all 700 of them. To learn more about PHP and all of the cool stuff that you can do with it, head over to the PHP manual at http://php.net.

Cascading Style Sheets is a standard language for defining the look and feel of text and other elements on a web page. CSS is what you will use to "style" a plugin beyond its original design. You can learn more about CSS from any of these great resources at http://delicious.com/popular/css.

JavaScript is another very popular programming language that runs on your browser—opposed to the server-side nature of PHP. JavaScript allows you to make changes to virtually any part of your website without having to tinker with PHP. For more information about JavaScript, check out http://delicious.com/popular/javascript.