First you need to perform the PCI DSS Self-Assessment. This consists of filling out a self assessment questionnaire (SAQ) (A through D) depending on how you are processing and storing credit card data. Then filling out an attestation of compliance.
A chart that shows this can be found at
https://www.pcisecuritystandards.org/sa ... fits.shtml
Instructions for completing the SAQ and determining which SAQ to fill out can be found here.
https://www.pcisecuritystandards.org/sa ... structions