If you are accepting Credit Card payment you will need to use a Credit Card Gateway. Examples would be Authorize.net and Mercury Payment systems.
*Note: You may notice "POS" as an option. Contact your POS Dealer if you are using an integration to see if this option is possible.
Setup your Gateway by contacting Authorize.net or Mercury. When the Gateway is setup plug your API Information in your Admin Control Panel under "Credit Card Gateway".
If you are having issues with Credit Card orders/Attempts check the documentation for the Credit Card Gateway you are using. Example (If you are having issues with Credit Cards and you use Authorize.net please view "Authorize.net Gateway Setup and Troubleshooting".