Certification is going to come in the form of ISO 9001 Standard or API Q1 8th edition Specification or ISO TS29001 (technical standard).
Certification means your quality system meets the requirements of one of these stated specs or standards. You work with an outside registrar to obtain certification for your quality system.
API Spec 6A has requirements for the product itself. API 6A Certification is going to require that you manufacture your product to the requirements of API (Specification) 6A. You can apply for a license to apply the API Stamp to the product.
If you have the quality system requirements down pact, then compare your system to the supplemental requirements found in API Q1. Next step is to review 6A to see if you are meeting the requirements for the manufacture of products to which this spec applies.
API has some very good liturature to help with the process of getting a license. Do not be afraid to ask them.