I hate to send you to public television to start, but I will. Go to the PBS web site and remember the reason public broadcasting exists. It started as EDUCATIONAL TV, and for once; on this web page it lives up to it's promise. The crash course will educate you, simply, on the history of television, and the future. Spend a few minutes here.
When you're ready to graduate, try How Stuff Works. This one is pretty cool. A basic, clearly written explanation of what's going on and what you need for HDTV. It also offers a collection of links from the technical to the commercial for everything HDTV.
HDTV Galaxy has some interesting information, especially if you're ready to do your HDTV homework. The usual collection of questions and answers, reviews of the gear you'll need, and even forums where you can ask questions and share opinions, including one special corner for any of us who're just starting out in the brave new high tech world.