Many cruiselines such as NCL, add the $10.00 per day per person gratuity to your on-board account. You don't have to worry about whom to tip, unless you choose to give extra to deserving crewmembers.
Room service is free. But, you might want to tip the delivery person a buck or two.
Always fill out the comment card at the end of your cruise. These are read by management. Your comments might determine which crew members receive promotions. Negative comments for poor service, or suggestions for improvements are also important.