To register for Salesforce.com certification exam you have to visit following URL:
On Webassessor you have to create a new webassessor login. After you have created a login, you can choose the exam and a location convenient to you (You can also take the exam online from your home in online proctored mode, if you have a webcam), pay the fees via the credit card to schedule the exam. You can schedule the exam based on time slots available.
Webassessor has testing centers across all major cities on the globe. You can chose nearest to you.