Congratulations ! You have decided to do something you will want to do over and over again. I have been on 14 Carnival cruises and I love them. Best vacation for your $$$$.
Make a list of everything you think you will need then take 10 things out. I start making a list a month before I leave. Crazy I know.
I take : Camera, extra batteries, new SD card for digital camera, large kitchen size garbage bag for dirty laundry, small ziplock bag and quart size ziplock bag for anything wet, sandy, liquid. I use ziplock bags for shoes especially if they are wet from swimming in the ocean and sandy. Ziplock bags are great for things like shampoo, liquid soap, perfume, q tips, bandaids, women's sanitary pads, medication in their bottles. I take a pad of paper, a pen, pencil, my nook. Be sure to take a power strip. There is usually only one outlet in the cabin. I take my Ipod, bathing suit, you do NOT need beach towels. They supply them for you and you can take them off the ship.
As far as clothes : You didn't say how long your cruise is. I take a pair of shorts for every 2 days, they don't get that dirty, a clean shirt for every day, 2 bathing suits, a pair of jeans, underwear, socks, sneakers, sandals, dress shoes for elegant night. I take 3 or 4 dressy outfits for the main dining room. The list can go on and on. What else do you want to know?