Hi C........I've been on a Carnival Cruise and a Princess Cruise.........Princess wins hands down.....but the Carnival one was fun too.....
Depends on where you are going............but as for the dress.........you will see everything from cocktail dresses, to very shiny nice pant suits......if you don't want to spend a bunch of money, get black pants/skirt and a couple of shiny, sequenced tops, maybe a nice little shirt under it and rotate it around......that will work.........some will be in ball gowns........other not.........so just something you like having your pic taken in and you feel that looks good.....
Speaking of Pics.........Carnival takes pictures ALL the time.....getting off and on at stops, during the dinners.....so be prepared to buy some.........they do great little critters on your bed.....out of towels, those are pretty cool.........
If you think you might get sea sick, get the patches from your doctor before you go...........I was raised in little boat, NEVER thought I would get sick, but I did..........and it's not cheap on the ship!!!!
Cruises are really great.......you meet great people, it's really cheaper than a hotel, rental car and eating......and they have great shows......try to make the shows and some of the games on board......they are alot of fun....
If you have the chance, go it's definitely fun.......good luck and have a blast.