Hey Carly. A cruise is fun! There is sooo much for you to do if you are a social person and not shy! Carnival Pride is a great ship and I know others have been on her and can tell you more about the ship.
I have been to Mexican Rivera with Carnival too and it was a blast. And you being 19, you can drink and party all night! You don't need to bring any fancy dresses, just something formal if you prefer. Just bring some warm clothing incase the weather turns bad, but bring tee-shirts, shorts, tanks, and don't forget a swim suit!
I hope that helps. If you have any more questions AIM me at daswimma100.