Just got back from a cruise with 3 nieces and a sister...and believe me all their suitcases were huge and they didn't wear everything they brought! There is a laundrymat on board, complete with an ironing board, you will need quarters!) Most of the time you will be in sweats or a swimsuit, depending on how comfortable you are in your own skin. Bring a formal dress and maybe one 'less' formal dress, depending on what you have signed up for. Loads of shorts, short sleeves, one sweater, hair thingies if you have long hair, sleeping aides, alarm clock, mini binocs, batteries, tip money if your steward is good (our's was awesome!), celebrating an event, bring tape, balloons, tapers...shoes for the shopping and learn a little about the language and learn to barter!
Of course if you are heading to Alaska, forget all the shorts and short sleeve shirts...bring a book if you like reading, cards. if you like solitaire, money if you like the slots. The shows are great if you want to go all day and all night. The cruise gyms are magnets for those that don't normally go to gyms...they are always busy. Pick a favorite crew member, stick with them and they will treat you right. Tip them well (I left a money clip full of money for our steward) . Photos are expensive!!! But have as many photos taken as you can, then you will have lots to choose from! My nieces were well received by the photographers!!! By the time we were done, our 'envelope' was about $2000 worth. If you are with a group, each buy some and then scan them, cuts the costs.
Don't forget sunscreen.
There will be singles groups, some you won't be able to attend. Believe me, the 'Ladies in Red Hats' group are a kick! ('A group of ladies over 50, international group, if you are over 50, I'd join!).
Jen, the best advice I can give is to not panic about things that are beyond your control. I just spent a week with a sister who spent most of her time worrying about her adult daughters and whether they were going to be 'there' on time. If you are free of that responsibility, you will have a great time! JUST REMEMBER! Bring fat clothes for leaving the ship! You WILL gain weight!
all the best,