You have to study for the exam, and then buy a voucher for it. Once you have the voucher, you go to a Micrsosoft-approved testing center in your area and take the test. You can get a list of developer exams here:
Google, as always, is your friend:
Visual Studio Certification[
^]
I am of the personal oppinion that MS certification is a waste of time and money because every time they release a new version of Visual Studio, your current/past certifications aren't applicable. It's just a way for them to get money from programmers who would actually be better off spending that cash of tools to help them do their job rather than meaningless certifications.