*IF* you booked directly on the Celebrity website, you can log in to your Cruise Planner, click on Manage Reservation, then Show Details if not already displayed, then click on Gratuities...should be highlighted in blue. Fill in CC info and you should be good to go.
Otherwise with TA as DaKahuna explains...
ETA: the above assumes you booked a cruise only non-suite cruise...you may also need to wait a couple days if you *just* booked it because it takes X a few days to completely process your booking...it's a weird process...