I wholeheartedly agree with the previous posts! Go with a Travel Agent that specializes in cruises. They are a wealth of information. As uncle Bob used to say "A Travel Agent Is A Knowledgeable Guide. An Experienced Traveler. An Invaluable Resource. A Fulfiller Of Dreams." Do some Agent shopping and don't be afraid to ask questions. Once you find an agent you like they can be of great value to you now and in the future. Ask around for recommendations for good agents.
To find a Travel Agency near you that is approved by Carnival look here.
However, having an Agent that is near you is not really all that necessary. I have friends that have an awesome Agent several states away from them. It is a good place to start though.