Carnival has something like 27 ships, each of which is at sea pretty much all the time. They have had some troubles recently, but I suspect they have fired a few people and cleaned up their act.
Here's the thing, you are going to have a blast. I'm not a fan of Carnival, I've done it, and it was fun, but there are better lines. That said, you don't have anything to compare it with, and most teenagers aren't that picky, so it will be great. There is food all over; you can always get pizza, ice cream, that sort of thing. The food is pretty good, and Carnival is a big party-party cruise line, as in constant action.
One thing, suggest to your parents that they check out the Chef's table dinner, for them not you. It's an extra $40 per person. I did it, and I'm a foodie. A meal like that in a regular restaurant would run an easy $150. They only have 12 people at a time, so if they are into food, recommend that they reserve it ahead of time.
Anyway, back to you. Go to the kids' meeting the first night. Even if you aren't interested in the programs, you will meet other kids, and clique off. My sons are rotten, and that's what they did. We're going on our seventh (my kids' 6th) cruise this fall, and we all love them. Bring at least 2 bathing suits, a decent shirt and pants for formal night (lobster!). decent walking shoes, flip flops. and decent shoes (for formal night). What I do is buy a T-shirt at every port.
Just chill out and enjoy it.