In order to practice you must first obtain certification. Once obtaining initial certification you earn the C in PA-C.

Certification is obtained through the National Commission on Certification of Physician Assistants (NCCPA).

Initial certification is open to graduates of an accredited physician assistant program by the Accreditation Review Commission on education for the Physician Assistant (ARC-PA).
To obtain initial certification you must pass the PANCE - Physician Assistant National Certifying Examination. This is a computerized 300 multiple-choice question test to asses basic and surgical knowledge.
This is a difficult test, but your PA program will help you prepare for is so that you may pass. Some programs will openly share what their "pass rate" is for the PANCE.