First of all...don't be nervous about anything at all. As long as you and your party show up at the pier on time with the appropriate boarding documents, you would have to try really hard not to have a great time. That being said, I would recommended that you try to get your excursions booked now, rather than waiting because the best ones do tend to fill up.
As far as motion sickness, since you are traveling on the Oasis, which is truly the state of the art, you will likely not have any problems with motion sickness. However, if anyone in your party is especially susceptible, or in case of rough seas, my advice would be to visit your primary care provider this week and ask for a prescription for Scopolamine patches. They are tiny, circular band-aid looking things that you stick behind your ear and they are not available over the counter. The best thing to do is to put the patch on about four hours or so before you set sail for maximum effectiveness. A single patch is good for three days and they typically come in a box of four. I know that some of the other folks on here have remedies that work, and I will let them tell you about their methods of choice. I can say that medically, scopolamine has been shown to be quite effective in combating motion sickness. Also, the patches are not cheap, so depending on whether or not you have insurance that will pay for them, you may want to make another choice.
Good luck and enjoy your cruise!!!