Carnival does not have any formal nights. They do call a couple of nights "elegant nights", and some folks do dress up, but slacks and a shirt won't be out of place either.
On non elegant nights they do even allow dress/bermuda shorts in the dining room.
So, basically, dress comfortably and cleanly; bathing suits and cover ups during the day; slacks, skirts, etc in the evening. No muscle shirts etc for the dining room.
The weather should be great, but it's never guaranteed.
If you are going to use a credit card to guarantee your onboard expenses, make sure there's enough available on it to cover those purchases.
The more you drink, the larger the end of cruise tab will be
Expect to have a ball on board. Shorter cruises will generally have a fairly young crowd on board, and everyone is busy trying to get seven days of fun into five. Enjoy!