1. All cruise ships are plastic, or in otherwords, they don't always accept paper money. Most bartenders will require your Sail and Sign card (Ship's ID card) to complete a transaction. You can however add money onto your Sail and Sign card by paying extra cash to the Purser once you have already embarked. However since you are 21 I would recommend a credit card, it just makes life that much easier, but it still does not make it any cheaper.
2. It is never necessary to bring formal clothing. One night onboard will be designated formal night, if you plan on eating dinner in the formal dining room, you do have to dress up on this night, however you can always eat at the buffet everynight in nothing but your underwear, if that's what you really want to do.
3. If you bring your own alcohol make sure to hide it in your checked luggage. If the cruise line finds it they will either throw it out or give it back to you at the end of the cruise, maybe fill some water bottles with Vodka? Anyway, there are plenty of cups and glasses onboard so don't bother packing any in your suitcase.
4. Carnival will automatically charge your Sail and Sign card $10 per day per person. 4 day cruise = $40 times the number of people. If you are paying in cash for your Sail and Sign card, make sure you have enough tip money this way you don't run out of alcohol money.
5. There is room service 24-7. The choices aren't that great, just your typical cold sandwiches, fruits, cheeses, and desserts. But its all free, however you should throw the crew member who delivers it a couple of bucks just to be nice.
6. Never book a room close to the elevators! Too noisy! Just try and get as close to the center of the ship as possible, in addition the least amount of motion is felt on the lower decks which are the cheaper decks anyways
Bon Voyage! -D
