In Store -> Payment -> Credit Card configure the credit card payment options. In "Select Card Types Accepted" click the checkboxes for the credit card types that you want to accept. In "Select Gateway Provider" select the payment gateway that your merchant account is compatible with. Enter all the neccessary information for the payment gateway that you select. When you finish click "Save" button.

If you wish to disable credit card payment and use only other payment method like PayPal or Google Checkout, select "Don't accept Credit Card" in the "Select Gateway Provider" drop down list.