No worries. As PC Guy notes, there will be two "formal" nights with the rest being casual. For the formal nights, a suit or sport coat and tie will do. Cocktail dresses are popular with the ladies. If you wish, a tux or dinner jacket is fine too although they are no longer in abundance - as they were years back.
Casual nights means pretty much anything except shorts, t-shirts, and on some runs they also mention no blue jeans. For casual nights I typically wear docker-type pants, golf shirts, tropical shirts, things along those lines.
And as PC Guy also said - on the first night they really don't care what you wear. Luggage service may not be completed and some people may still be wearing whatever they had on at boarding time.