In many cases, tips are automatically charged to your purchases, however, tipping is encouraged for outstanding service from stewards, waiters or other service people. This is how they make their living, so if they are doing above and beyond to make your cruise memorable, feel free to tip. Our room steward was AWESOME! So we would tip him just about every night when he did his turn-down service for us. Carnival also will leave you an envelope on the last night of your cruise to tip your Main dining room Matre'D. Although he does not do any of the serving in the main dining room, he is responsible for making all the servers and dining experience the best it can be, so in essence, he most definately plays a "behind the scenes" role in your main dining room.
A ship can be a place where you can feel bombarded by "extra" costs. Depending on your budget and the types of things you want to spend your money on, you can adjust your cruise habits accordingly. With drinks, we really found it convenient to get our non-alcoholic drinks (iced tea, lemonade, etc) from the informal dining room dispensers (which are free). On the pool deck, there is always a pitcher of ice water & glasses to serve yourself. If you are a big soda drinker, you might want to consider purchasing a fountain card which will give you an unlimited supply of soda pop from any of the servers, bars or dining rooms. I did notice a handful of people bring along a case of their favorite soda as well to supply their craving for the carbonated stuff. We really didn't order alcoholic drinks much, but they can really beef up the balance on your sign n' sail card if you're not careful.
Hope this is helpful! If you have more Q's, just ask. We were first time cruisers, but learned a TON from the folks on this forum.