Welcome Mcdowell,
You can start by visiting the Carnival web-site and look at the different ships and itinaries and find the one that interests you most... Once you have it narrowed down, using a good travel agent would be the way to go.. When do you want to cruise, soon?
A good idea is to also read through the section here on Cruisemates for first time cruisers.... Do you have an idea of where you want to leave from?