The meals in the main dining room and buffet are included in your cruise price. If you make reservations for a speciality restaurant, that is extra and its not all you can eat, although I highly doubt you will be hungry after one of those meals.
As for room service, its also included, but you'd need to tip the person who brings it a few dollars when they bring up the food.
While on the ship, other than some minor tips, all your charges go on your sign/sail card and will be charged to your credit card, you don't need much cash while onboard.