You shouldn't have to make a special effort to prepay the gratuities as they are now virtually always automatically charged to your shipboard account. Figure it will be about $10 per passenger per day.
As far as suggestions of things to take, there is a packing list accessible through our sister site, cruisereviews.com. Personally I'd suggest a travel alarm clock (windup or battery best), an extension cord, and perhaps a small flat plug-in nightlight - especially handy if you have an inside cabin! Oh yes, something you won't find on any list, but is essential: the attitude that you will have fun, no matter what happens!
Picky eaters will probably find plenty of good choices in the dining room as well as the buffet. Try both, but don't skip the dining room. Between about 5 different entree choices on the menu (plus a few standards not always listed) - not to mention all the choices in soups, salads, and desserts - even the pickiest eater would have trouble going hungry!
Have a great time!!