A quick start: http://www.w3schools.com/Ajax/Default.Asp

Then : http://ajax-tutorials.com/

And many links for ajax:
http://www.ajaximpact.com/tutorials.php

And little more here...
http://www.ibm.com/developerworks/web/library/wa-ajaxintro3/

See and learn something from here as well.

These were the sites from where i learned it before. Though these all you can find in google as well