Hi jrnygrl,
I have been on three cruises. It seems that on formal night most men have been wearing dark or buisness suits. Many women wore gowns but there were quite a few cocktail dresses as well. On Carnival, formal night seems to be the night that they have the meet the Captain reception and the big Las Vegas type shows. Some people like to really dress formally for the evening with tuxedos and bow ties. I have even seen couples where the lady was in an evening gown and the gentleman was in a business suit or the gentleman was in a tux and the lady was in a very elegant black pants suit with stunning accessories.
If you lack your own items you can always rent or buy from the ship.
This will be your vacation. However, in order to fit in and not feel self conscious, be prepared to feel elegant on formal night. You won't regret it. Most people in the dining room will be conforming with the formal code for the evening. However, most of the sophisticated guests are very tolerant to a degree.
If you don't feel like dressing up on Carnival you can always opt to dine on the Lido deck where there is always informal dining.
Personally, I like the diversity of the evenings. As a man, I wear a black suit, white shirt, long tie, and my best pair of black shoes. I actually go and change to smart casual before the big show so that I can feel really comfortable.
But most of all, please, please enjoy your cruise. After your first experience, you will be an old pro at cruising. One learns so much on the first cruise and there are so many great people willing to help.
Have a great time.