You need to get "Merchant ID" and "Transaction Key" from Cybersource. To get "Transaction Key", log in to the Cybersource business center. Click left navigation menu "Account Management" -> "Transaction Security Keys" and then click link "Security Keys for the SOAP Toolkit API". Click "Generate Key" button to create "Transaction Key".
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 "Cybersource". Enter "Merchant ID" and "Transaction Key" and check "Send To Production Site". When you finish click "Save" button.