That isn't a dumb question.
Go to the Carnival web site. Click on gifts and services. Click on Fun ship dollars. Buy as many as you think you will need for your on board expenses. They will then give you a place to add your address to mail them to you. Instead of your address, put On board credit, Room number. You pay for it with your credit card, and Carnival will send you an e-mail thanking you, and it will say on board credit. Print that out and take it with you to the ship. If you do it soon enough, it will even appear on your Documents when you get them in the mail, or by e-docs. If you do not use the whole amount that you have purchased, you will receive a check on the last day of the cruise, and they will even cash it for you.