Yeah, I would agree with PPs that Carnival is a bit more of a party type cruise, from what I've heard (my sister has cruised on Carnival, Norwegian, and Princess). I've cruised twice with Princess and absolutely loved it.
I have only been on 1 cruise and it was on Royal Carribean Allure of the Seas. That boat was magical and huge lol. I did hear tons of horror stories from other people on the ship about Carnival including bartenders slipping drugs into girls drinks, horrible tasting food, food poisoning etc. Which ever you choose or find the best deal on, do your homework as I am sure all of them have good and bad things happen. Read reviews but a cruise would be very fun!
My fiancé and I love cruising. We have been on about 5 cruises and are going on are going on another cruise this upcoming October. I've been on both Royal Caribbean and Carnival and have had amazing experiences on both. I really think your vacation is what you make it too though. There are so many opportunities on cruises to be outgoing and really participate in different activities. It can be a lot of fun and you'd be surprised at how many friends you actually can make in a short span of time. With either cruise line just make sure you are taking a look at the ship you are booking on. If you go on YouTube you can see plenty of live ship tours to give you an idea of what the amenities are like. When possible I would book a new and or renovated ship!