I am a single mom that cruises with my girls (about your age).. the things we cant live without are walkie talkies.. it allows them to go off and explore and still keep in contact with me. Highlighter for the capers, lanyard for the sign and sail card, good waterbottles - we fill them with ice and water on board and bring them to the islands. A good waterproof wallet so that you can have your money/credit card/sign sail card with you at all times at the beaches. Also dont forget a variety of medicine (just incase), powercord to plug different chargers at the same time.
As for clothes.. my girls wear nice dresses for formal nights, and either capris or skirt with a cute blouse other nights.. typically we bring both black and white sandals which will work for all nights. most days you will be in your bathing suit.. they pack at least a few bathing suits and coverups and comfortable flipflops (or crocs) that they wear around most days.
Take advantage of the kids (or preteen) clubs. You will meet lots of other kids your own age and it will give your mom time to enjoy some adult conversation. =)