Start from here...

http://www.asp.net/ajax/

Study this book:  Wrox.Beginning.Ajax.with.ASP.NET.Sep.2006

I have pdf version..if u want I can email you...

Good luck

Just to get started you can just google it and follow. But if you are implementing some advanced ajaxy user interface, I would recommend using some javascript frameworks, eg, jquery dojo prototype etc...this way you will do away with browser specific js and use apis for dom manipulations event handling etc...

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