First of all, it might be your mistake to develop back-end separately from front-end, as it's easy to make many mistakes in developing of the back-end if the development is not validated and tested on a regular basis using some more or less realistic front-end. However, maybe I did not understand you; and you did enough front-end prototypes. At least, it sounds a bit weird that you planned using AJAX but did not try it before your back-end is finished.
Now, how to learn? Fist steps would be using Google an Wikipedia:
http://en.wikipedia.org/wiki/AJAX[
^],
http://en.lmgtfy.com/?q=AJAX+(manual+OR+tutorial)[
^].
First thing found on Google is already promising:
http://www.learn-ajax-tutorial.com/[
^].
First tutorial of the tutorial kind references by the Wikipedia also looks competitive, because the tutorial is very short and would help fast start-up:
http://www.xul.fr/en-xml-ajax.html[
^].
Don't you think that this method of "asking" is more effective than asking just CodeProject?
—SA