I'm not sure you'll get much of a choice. Unless you are in the entertainment industry where you can be a DJ or a pianist or something, or have a specific skill - daycare, beautician, bartender, you may have to take what you are given.
But, if you are young and enjoy working with people, I think it would be a great experience.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007