Judy, my wife and I sailed on the Destiny last year, and it was our first cruise. Best advice--keep track of the bar bills as they add up fast! Dine in the main dining room each night, as the menu varies and it is exquisite. The water slide is fun, and if you like to lounge around the pools, nab a lounge chair early. As for the rule of "no leaving personal belongings on chairs to reserve them"--forget about it! Everyone leaves stuff on chairs first thing in the AM and return later to use them. Also, if you like art, go to the art auctions. You don't have to buy anything, but they give away nice gifts to all who attend at the end of each auction. When tipping, give your waiter their tip AFTER your last dinner on board, not before. Service is bad if you tip too early. You will have so much fun, and it doesn't matter where you are from.
PS. we are taking our 2nd cruise this December on the Grand Princess, but Carnival was a great 1st cruise.