My parents are bigtime cruisers... they take at least two per year and have cruised all over the place... The only time they've ever complained of the boats getting rocky was when their ship cruised almost into a hurricane.. they got bounced around a little but other tha that - it's stable... and my mother is a big ole pansy...
Why my parents are both senior citizens, they have taken most of the major cruiselines but are partial to Carnival because as they put it - "it's not all old people" they can't stand being around old people...
Depending on where you're coming from.. sometimes the airlines have packages with the cruise ships, other wise you can go directly to the cruise line.. or look at one of the brokers, like priceline or others that have discounts.
I would go crazy in an inside cabin.. so I wouldn't reccommend those - it'd be way too claustrophobic.. and a lot of boats now have balconies which are really nice... If you get a cabin in the middle of the boat, youd have the least chance of getting any kind of motion sickness even if you are sensitive to it.. (plus dramamine works pretty well)
Top ships for teens might answer some questions about what teens rate hightly on crusies