Before starting with Ajax + asp.net Application you need to do follow.
at first you know how to configure the ajax framwork to your asp.net application
1. configure ajax framwork on asp.net application
2. Sample code
4. ajax + .Net Video
5. and if you have any question please ask Here
6. AJAX Application Architecture
7. ASP.NET AJAX 4.0 Preview 4
hope it will helps you.