I agree with the PP.... check out
http://www.cruisecritic.com for lots of great advice and information.
If you have an inside cabin, take a nightlight. Also, most ships don't have alarm clocks, so if you like to check the time, an alarm clock is invaluable. Check to see if your ship has a self-service laundry. If so, take laundry soap and dryer sheets.
Pack your own OTC medecines... things like cold medecine and Tums are expensive and do sell out on board the ship. I try to pack a full arsenal of medications just in case: anti-diarrheal, Tums, decongestant/expectorant, Tylenol, sun lotion, etc... I pack the tablet versions because they weigh less and take up less room.
If you're flying, pack your clothes for the first night's dinner in your carry-on. You probably will get your luggage in time to change for dinner, but not everyone does.
Do your research (cruisecritic.com is invaluable for this) on what activities are available and what they cost at each port. If you want to go to a beach and relax, it is often much cheaper to get off the ship and grab your own cab to go to the beach than to book it through the ship. Also, some islands are great for activities like snorkeling and others are not. You will be much happier with your shore excursions if you do a little legwork before you get to the ship.
Set your own speed. Do the things that sound like fun and interest you and ignore the rest. Don't try to do everything.
Enjoy!