I have done a fair amount of traveling in my lifetime and several of those trips included cruising on the high seas, so when it comes to packing and planning, I would suggest that I have a fair amount of experience in this exercise. The most important thing to remember when you pack your suitcases is that you will not be near to your home so you will not be able to go back and pick something up that you forgot. So unless you plan on spending a chunk of your budget at the gift store or at the various tourist traps on your route, pack appropriately.
One of the greatest advantages of a cruise is the fact that it is usually all inclusive, which simply means that you will not have to pay for food. First time cruisers usually get stuck with unexpected charges due to the fact that they did not budget for alcoholic drinks (heavily priced) and shore excursions (also heavily priced but are usually good deals). The problem is that passengers find themselves caught up in the moment and end up with a smoking credit card because they rationalize these purchases as a once in a lifetime opportunity, forgetting that there is always next year. Remember that alcohol can be purchased on the island (much cheaper) and consumed in your room and you don’t always need to pay for an excursion, sometimes the only thing you will need is a good map.
The only thing left to consider is the documents you will need. In most cases you will need to have a passport and sometimes a visa. When traveling on a cruise ship visas are not necessary, however a passport is required if you are leaving from the United States. Remember to communicate with your travel agent beforehand about your travel documents, especially if you need time to apply for a passport. Once you have figured out what to pack, the only other advice for you is for you to have fun.