Be sure to get a balcony room and if you can afford it, get a suite. The extra space is nice to have.

I've been on Royal Caribbean a couple times and Carnival a couple times. I think Royal Caribbean is a little nicer. My wife has been on Disney and she enjoyed it.

When you pick a destination, research the excursions you are interested in. You can usually get better prices on the excursions from companies not affiliated with the ship. Research them and read feedback from prior users.