One of the best sites for a beginning photographers (as well as pros) is Cambridge in Color. This excellent site has some superb writers, and their articles are clear, concise, and thoroughly demonstrate their points with sample images, figures, and some basic math. You can learn a lot there.

Another great beginner site (and even for those who are decently skilled photographers) is Digital Photography School. This is a great site that is chock full of tutorials, tips and tricks, articles, and more. You can find an article for just about anything related to digital photography there, from choosing cameras, to learning about exposure and camera settings, to composition, to post processing techniques.

There are also numerous other sites on the net for beginners and experts alike. If you like the nitty gritty low level theory about everything photography, another excellent site is the Luminous Landscape. The articles here are often fairly old and pertain to film, but the underlying theory is sound, and most of it still applies to digital photography today. Be ware, while this site has some fantastic articles regarding composition, exposure, etc, it also contains many heavy duty articles that discuss the low level theory of optics, film, sensors, light, etc. If you like mathematics, its a wonderful site. If you hate math, you can avoid the gritty articles and stick to the ones that discuss practical things like composition.