Celebrity cruises are the best!! i went on the constellation around the Baltic sea this summer and it was sooo good! the food was great! and i'm 16 as well and what i did was go to the first teen thing on the first night and i met like 4 people already who decided to meet up again to go swimming the next day and then after that day we met like literally 100 people between about 15-22! just picked them up along the way hehe, most people just approach u and start talking to u.
activities wise, just make new friends and hanging out with them is enough, u will find anything on the ship is entertaining as long as you're with friends. and spend as much time as possible with them, u always find its not enough by the end of the cruise!