Hi Samantha,
How fortunate you are to be able to take a cruise so young! For onboard activities, really study the daily "newspaper" the ship will give you each day. There are lots of on board things to do. Also check in with the youth center since they will have actvities planned for folks your age. As for "off ship activities" referred to as "excursions", you can check out
www.rccl.com,
www.celebritycruises.com,
www.ncl.com, etc." for the excursions that are offered. Most cruiselines offer the same set of excursions (Princess cruises don't list their excurions with write-ups on the web). As to what to bring, don't forget warm layered clothing, rain gear and binoculars. There will be plenty of nature to take in on this trip, but you'll miss a lot of it without binoculars. Have fun and when you return let us all know how you enjoyed it!