Mastering Ajax
Understanding Ajax, a productive approach to building Web sites, and how it works by Brett McLaughlin is a pretty decent introduction for people who don’t really know where to start. What, why, how and all that…
Or you could scroll down to resources and simply get to work.
Just don’t overdo it. No matter how useful the ajax approach is, it’s not really necessary sometimes. Think flash intro and you’ll know what I mean.