The tip will automatically be added to your account (you can adjust it as needed at guest services, or add more cash to the envelopes at the end if you wish.) The standard gratuity amount is $13.50/cruiser/day...if you have 4 people in your room, you'd pay $54/day in tips. The $13.50/cruiser/day breaks down as $4.50 for the stateroom host, $4.50 for the server, $3.50 for the assistant server, $1 for the head server. As others have said, you'll get slips/envelopes to give to the 4 people towards the end of your cruise.
The only other tips you may want would be some cash for the free room service (we do $1/item ordered), and tips for adult dining, (and maybe for luggage handling in the port). Any drinks ordered from the bars already have a 15% tip added to them, and spa services already have a tip added to their totals. The auto gratuities are meant to cover ALL your regular dining on the ship--breakfasts/lunch/snacks/dinner, etc--doesn't matter who is helping you at the other locations.