S.M.
Girl - you will have so much fun! Everyone will! We cruise each year. I don't have time to go in to detail, but I highly suggest you go to http://boards.cruisecritic.com/ and peruse the boards. There is a board for Carnival, a board for 1st time cruisers, boards for any ports of call you might have...you name it. I will try to post back with some packing lists when I get a chance.
As for food - like a PP said - you don't need to bring anything unless someone HAS to have a certain food or they are allergic. Even then, the ship is very good about working with you.
Check with Carnival's policy about bringing drinks on board. We have been fine with drinking the ship water (including my then 10.5 month old girls)...but everyone is different! However, bottles of water are expensive. Some cruise lines will allow you to bring drinks (water, soda/pop, alcohol, etc.) onboard in your suitcase.
find out if there is a "formal" night and then determine how dressed up you want to be for that.
If you stop anywhere, see if there is an excursion - good excursions book up fast...but you can also do your own thing too (for cheaper).
Are you going to the caribbean? You just need warm (80 degree) weather clothes for on the ship. I wear the same clothes on embarking and disembarking (I know - not everyone is big on this)...try not to pack too much though - really. don't forget your bathing suit. "maybe" a light jacket for night...but I've never needed a jacket.
update: there are many cruise packing lists out there, but here is one I'll throw out to you: http://cruisediva.com/pack_it.htm (on the side you'll see lists for men, women and children). Its very easy to go overboard. Try to look at the list and think "do I REALLY need that". Like t-shirts - do you plan to buy one on board - then pack one less.