There will likely be 3 formal and formal is wearing dark suit, there will be many wearing dinner suits or a tux. On the informal nights a jacket/trousers/shirt/tie whilst Cunard does not have casual nights but only elegant casual ones, on these night shirt and tie but not normally a jacket but there is always a variation.
Long evening dresses are the norm on formal nights but many people also wore evening trousers and glam tops. The shop on board also sells evening and day clothes. Surprisingly, I saw the same top I had bought this Summer in UK but because the prices are all in dollars on the ship, it was about 1/3rd cheaper.
I also found that for the informal nights, good restaurant/theatre outfits were ideal. Daytime attire would be the same as in a smart hotel. Would suggest you take a pashmina as sometimes the Med area can be quite chilly at night. You will have much warmer weather than we did in the Fjords, so you will certainly need 'poolside' swimming gear and cover-ups.
N.B. We had heard that there was little drawer space in the cabins but we found that the shelves in the wardrobes were OK and I took large zipper bags for underclothes, socks etc. The space under the bed took our 3 large suitcases, plus the life jackets, so we were not inconvenienced at all.
One tip: If you like decent coffee, the bar near the Brittannia restaurant on Deck 2( I think it's called Caronia ?) has a decent Italian coffee machime. You can buy a capuccino 'to take away' and then go up to the Lido for the self-service breakfast (although we never tried to take it into the Brittania restaurant)!
It wsa also our first cruise and we hope you enjoy the experience as much as we did.