You should have an implemented quality management system before you can ask a certification body to certify your quality system. At a minimum, you need to have performed a full cycle of internal audits and a management review. It is advised to request different quotations from different certification bodies to make a good choice.
The website of the Council for Accreditation (www.rva.nl/search/) may help you to find all certification bodies for ISO 13485 certification.
Many certification bodies will start the certification process with a documentation review. In this phase, the documented quality management system will be assessed for compliance with the standard requirement. If not, then adjustments need to be made to become compliant. In a next phase, the implementation will be verified. In other words, do we live up our own quality management system? This might result in some deviations found during the audit which needs to be adjusted as well. Once all significant deviations are solved, the certification body will issue an ISO 13485 certificate. The certificate is usually valid for a period of 3 years.
Surveillance audits will be conducted every year to see if your organization still meets the requirements of the standard and its own quality management system. A renewal audit will take place each 3 years to extend your certificate.