We are set to sail in March on the Carnival Legend. We have never cruised before, and don't know what to expect. We will be with our two children ages 2 and 4. Any tips/suggestions for cruising with young children. I am really nervous as cruising seems to be a whole culture I do not understand... yet?

Do I need to book excursions and childcare before we get on the ship (now?) or can it wait until we are on board? Any help & suggestions would be much welcomed!
Thank you!!