Welcome to Cruise Reviews forums!
Two things new cruisers tend to do is 1) overpack and 2) not realize how onboard charges can quickly pile up.
The ship is a cashless society where your Sign and Sail card is your cabin key, your ID, and your purchasing card. Keep track of how much you are spending on drinks, ship souvenirs, gambling, etc. Don't wait until the morning you leave the ship to discover you ran up a bill you weren't expecting. Also realize that Carnival automatically adds crew gratuities to your account. Trust me, they more than earn them.
Most new cruisers bring probably twice as much clothing than they will use.