After looking around in the Internet, I was almost lost. There are tons of resources that are interesting and useful, but I don't know where to start. Moreover, time is short, I can't take my own sweet time to sit down and study from the beginning. I need some intensive course to let me start working on my current project. Many resources are not related to ASP.NET too. Though there are ASP.NET Ajax Learning Video, the videos are for the Ajax Controls, and there's no sound for the video clips, so I can only guess and watch the codes. That doesn't help much in the introduction and explanation. Moreover, I think it's time consuming to watch the video tutorials than only reading text. I also downloaded the Ajax Sample Applications, but I haven't find it useful to me, coz I need more explanation.
Finally, after two day searching around and reading, I've come to this site - Microsoft ASP.NET AJAX documentation. I guess this is where I should start and move on, at least I found an Ajax sample application that I can follow what it teaches me to do.
Well, it's time to stop wandering around and stick to this, and go! ^_^