My advice would be to pre-pay for as much as you can afford or want to do,i.e. Pre-Paid Gratuities (Waiter, Room Attendant, Head waiter, etc.), cruise insurance (just in case you have to cancell too far along before the cruise) and any and all shore excursions. Should any speciality restaurants be available (Chops, Portifinos, etc) can usually also be pre-paid on RCCL's web site using your reservation number. EVERYTHING extra that you purchase ONBOARD the ship, which includes everything I mentioned above, will be charged to your Sea Pass card, which is, in effect your credit card for the ship. That Sea Pass card is linked to the method that you have chosen when you board the ship at the port, to which the cruise line charges to pay for everything you did ON the ship. Like most first time cruisers, you will most likely buy photos and mementos from the ship's store. Shop CAREFULLY. IF you are going to a port where you plan on going to some beach, I'd buy (at Walmart or such) "Water Shoes". Many beaches in the Caribbean are NOT like local beaches in the States, with lots of sand! Many beaches are very rocky once in the water (Labadee, Cozumel, etc). I'd safely say that almost every show, child's adventure group, ice skating, swimming, rock-climbing are free. Spa treatments, etc., are NOT! Once on the ship, check the daily cruise sheet that will be put into your cabin. Everything that is happening on the ship, almost minute to minute, will be listed there. Just in case, I always bring a couple hundred dollars cash to use when you are off the ship shopping, and in case you want to tip one of your servers or room attendant a little more for extra good service.
As a first timer, you WILL make mistakes. ASK questions of anyone here, or on the ship if you don't understand something. THERE ARE NO STUPID QUESTIONS, here or on board. Here, you will get many different opinions; none are any better or worse than another. Read them all, and then choose the ones that fit YOUR needs. And, as we always say, do one crusie, and you WILL be hooked. Have a great time, and keep the questions coming.