Congratulations, you made a great choice for your first cruise experience. We love Celebrity. I think you will be fine on any dining night. The clothes vary drastically even on Celebrity. As the last poster mentioned, informal night (which is a bit of a misnomer) is trickier. Many opt for shirt and tie, others jacket with shirt and no tie. Still others will dress the same as for formal night. Don't sweat it, you'll be fine. As far as over or underdressed, I don't think its possible to be overdressed for formal night, but I certainly felt underdressed on our first cruise (Celebrity). If you go to Celebrity's website they list the dress code expectaions. Any clothing lists I have seen on this board have been crazy, including extra panty hose for the lady and lots of black socks for the men!
As to the duct tape, forget it. Its one of those things you keep hearing about and I've never seen anyone need or use. Must haves: anti-nausea medication for your first day out (take before sailing just in case its rough) and possibly for other days if needed. I find once I have my sea legs I don't need it anymore. Gym/work-out clothes if you're into fitness. Refillable water bottle. Backpack for shopping or excursions. Good book or two. Most important is a great attitude that says 'we are going to have an awesome holiday regardless of weather, tablemates, or anything else that is off of perfection'. Add a friendly smile for your fellow travellers and definitely for the hard-working staff and crew on-board and you will have a great time. Happy sailing!