I highly suggest a TA especially for your first cruise. They can help you learn about some of the things that a first timer would not think to ask about. Mine got me a better airfare, and travel insurance rate than booking through the cruise line.
Determine where you want to go, and then research. This board has a lot a great info, and great people more than willing to help with your questions. Remember, no question is Silly, if you don't know the answer.
Watch for cruise bargains and upgrade opportunities too.
Independance of the Seas
Monarch of the Seas, October, 2007
Serenade of the Seas June, 2006
Now see when I looked at all the cruise books, I thought RCCL was much cheaper than Carnival. Ofcourse like it was said, it depends on the time of year as well. I did find out that booking early is much better in the long run. For the same ship, departure date and cabin catagory we paid about $240 less than the price the they have listed now. Quite a jump in price since Feb.
Sovereign of the Seas 5/07
Carnival Fascination 4/21/08