Carnival adds $10 per person/per day to your bill. If you choose to pay your waiters, etc. in person, I believe you can ask Carnival to take it off your bill and handle it yourself. But they spread it around to your room stewards, waiters, etc. and I suspect something in a pot for everyone else.
You may want to use that pricing as a guide, and what else has been mentioned here.
In addition, early in my cruise, I will give my room steward $10 - $20 and split up a similar amount for my waiters in the dining room. Then if they have been good to me, offer something additional toward the end. That way if I need something extra in my room or want to order an additional entre I find they are receptive to listening to my requests and friendly.
Tipping is an adult trait. They often are "stiffed" by students, so if you offer a little something extra to let them know you respect them and their work, it will go a long way... and you never know when they can do that something extra that makes your experience special. They work hard and depend on tips to make a living.
Have a good time, be respectful of the other fare-paying passengers, and enjoy the cruise. It is a great way to travel!