Start here :)
What's this AJAX stuff I keep hearing about?
JSON? I have a friend named JaSON.. does that count?
I've heard of something called a jQuery.. what's that?
How do I start!?
When it comes to web guides, there's no beating Tizag.com. But W3Schools.com has some good primers as well -- so take a look at both, and go with what you like!
A boatload of jQuery tutorials by awesome people