Hi Max,
You need to decide which asp.net exams you are taking 2.0, 3.5 0r 4.0. There are different certificates for each
see
http://www.microsoft.com/learning/en/us/certification/cert-vstudio.aspx
Then book your test at a Prometric Centre near you. It is computer based and can be taken at any time a seat at a test centre is available.
http://www.prometric.com choose Information Technology and Microsoft.
The price varies depending on Country.
Good Luck,
Ozzy Geoff