My two teenage daughters met alot of other teens, believe it or not, at the outside ping pong tables on our last cruise. You often had to wait for a table and this led to conversations and playing against each other, etc. They ended going to the teen clubs together and hanging out throughout the cruise (and naturally my older daughter was involved in her second cruise ship romance!)
I haven't participated in the teen club/hangout but next time I think I'm going to give it a try because that seems to be the #1 place, so I'd start there. Also, hanging around the Lido Deck watching the various activities, and then there's the real virtuality (video games) where I met A LOT of people
Just put yourself out there, because you're never going to see 99% of the people ever again (which can be sad if you do make friends), so don't worry, and just be yourself.