I know how you feel. I just came back from a cruise and I was 21 and my last cruise I was a bit younger so I had the teen club or other activities to go to meet other people my age. I basically went to the nightclub, hung out by the pool and participated in the corny and wacky activities and met people my age or close to it. You have to be outgoing. If someone doesnt like you or if it doesnt work out then who cares you wont see these people after the trip lol.
Don't worry. Be yourself and have fun =]