I am not a teen, but my girls went on 3 cruises with us as teens and had a GREAT time. you will meet lots of teens and they have a CLUB O2 just for teens on most ships. Do not be shy, look at he Carnival Capers and they will tell you the activities for the teens.
I also think thee is a meet on board section that you can find out who else is going. Also Carnival on Facebook.