How long is your cruise? A 7-day cruise has two formal nights. 4 and 5 day cruises have one formal night.
About the only things that don't cut it for casual dress are shorts and t-shirts, which are expressly not allowed to be worn at dinner although one occasionally sees them. This is particularly true on the first night since some baggage may not have been delivered to cabins. Blue jeans are also listed as no-no's but are worn by some and no one objects to it. I have been guilty of this last one (on Carnival).