OP, first of all, welcome, and congratulations!
You've made a choice (hopefully with the assistance of parents, etc.) and you are going to "start losing weight."
By the time you get to your cruise you will have many advantages:
1) You will already have begun altering your eating and exercize habits and developing good habits. That will make it easier to keep it up during the cruise.
2) The cruise is a great opportunity to be active. No cars. No Internet. None of your home gaming consoles or other things that might keep you inside. Plenty of time to go out and have fun, and at the same time be active. If you have a device that tracks your steps, I think you'll find you have no trouble keeping them at a high number daily.
3) Attitude. Remember, what happens to the scale after a one week cruise isn't what matters.
4) Not to be preachy, but keep in mind a healthy weight is important, but your weight doesn't define you. Remember the good things about the person you are while you are adjusting your weight to where you and your parents feel is best for your long term health.
I have a teen-aged son who is a bit heavier than he wants to be (his doctor agrees), and he is working on eating less and exercising more, but at the same time, I try to make sure he knows that he's a great person, regardless of his weight.