You have nothing to worry about. You will see every type of clothing. Dress for comfort and bring your fat pants for the last days!
Formal night is very dressy, accessories will jazz it up. Just you two being young and the smiles of "the fist cruise" will make it work. You will be the ones pointing fingers, you'll see. That's the beauty of a cruise, relax, enjoy and EAT! You kids have fun.