Credit Card
If you're using automatic credit card processing, then Credit Card will show you the authorization information from your online processor.
Processing Manually
Process the credit card like you would a phone order. For tracking purposes, enter the approval code and invoice # from the credit card transaction, and then change the status to "Processed". You can make any notes that you'd like to see for future reference.
If you're unable to successfully process the credit card, then contact the client for additional information. If you're unable to resolve, then mark the credit card has "All attempts failed", and then click on the GC and Void it.
What about the card security code?
If you're processing credit cards manually, unfortunately you don't get to use the credit card security code (the extra three or four digit number). The credit card companies specifically prohibit collecting the security code and storing it anywhere.
Once you're selling enough online, you should consider using online processing, which will save you time and get the slight reduction in credit card fees from having the security code. Learn more in Setup Basics.