Good choice of line. Carnival has always had a reputation as a party line, and to be honest, as much as they've tried to shake it, I don't think they have. Be warned however, that you must have someone over the age of 25 in each room that has anyone under the age of 21 in it. So if you're taking some of your younger brothers, be careful or you could end up with a mess at the port.
As far as places to party, there really is only one nightclub onboard, so nightlife usually centers there. Except for the night of the deck party, where everyone starts on deck and then gravitates towards the club. The casino is usually another popular place to start. As far as the other bars, I can't say they get too popular, but a bar is a bar. If you want to drink, hit the bar
As far as daytime, you probably want to stay near the pool deck. If the crowd is particularly good, you could probably get some games going (flip cup, pong, etc...) if you're all willing to throw down.
Next thing, pricing on ships isn't exactly cheap. Your best bet are the bucket deals.
Last, drink responsibly. You're on a moving ship. The rocking and rolling are going to affect you, and stay away from the railings. Last thing you need is dealing with someone going over. Otherwise, have fun!