Congratulations on your first cruise!
My wardrobe for the evenings usually consists of the following. For formal nights, I'll wear a gown, old bridesmaid dress, or cocktail dress. For casual or informal evenings, I'll wear a pantsuit or a dress -- similar to what I'd wear to my job in an office. In warmer climates, I'll wear a sundress, but have a shawl with me in case the air conditioning is too cool.
I tend to be on the dressier side, though. Pants are fine on any evening, and I have seen capris on the casual nights. Also, the longer the cruise, the more formal the dress tends to be. We women have a lot more leeway.