Check your library for books like the "Unofficial Guide to Cruises". These give an overview of the major cruise lines and what a typical (whaterver that might be) cruise for that line might be like. Some provide incredibie detail on each ship in the fleet as of publication.
Whatever you choose - enjoy your cruise.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015