For initial research I recommend
www.vacationstogo.com as I find it has the most detailed search facility
Once I have found a cruise I like the look of I go to
www.cruisecompete.com which lets a lot of agents compete for my business by offering the lowest price quote - but beware of offers to give you $500 worth of on-board vouchers as you won't actually want to use many of them
Certain cruise lines are aimed at younger crowds - Carnival and Disney, obviously - and others such as Holland America Line (our favourite) tend to attract older people. Likewise, the bigger ships have more activities so tend to draw more families with kids
As others have mentioned, Christmas is an expensive time to go cruising and a lot of the better trips may already be sold out