I read an article that cruises are still at near 100 capacity despite the economy. We usually book early but you can get a good deal by booking at the last minute. Although booking airfare at the last minute is a pain.
http://www.cruisecritic.com, reviews of ships/cruises.
http://www.cruises-n-more.com Decent search engine, some specials
https://www.kayak.com/cruises I love Kayak. I use it for flights all the time.
I would recommend Royal Caribbean out of Miami to the Caribbean as a first cruise, Lots of sun and fun. Southwest flies into Fort Lauderdale pretty cheaply, and they don't charge for luggage. Either book the cruise directly through Royal Caribbean or use Travelocity; both work well.