I presume that you would want the certification on the latest version (4.0) of ASP.NET. In this case you have to sit exam
70-515:
http://www.microsoft.com/learning/en/us/exam.aspx?ID=70-515&locale=en-us
If you follow the link above you will find the exam description and the skills measured by the exam. There is also a tab that explains the preparation materials available.
Once you are confident enough about knowing the subject matter, you can sign up for the exam. Go to
www.prometric.com, select "Microsoft" as the exam provider, and follow the steps on screen to sign-up for an exam until you find an examination center that is close to your location. At this point you may choose to
call the center directly to sign up for the exam, or to continue clicking "Next" inside the web site to register over the web.
