On a 7 day cruise there will be two Formal nights. Men can wear anything from a sports coat (you can go without, but most men will have a jacket of some kind on), shirt and tie and slacks, to a tux. Women can wear anything from nice slacks and a "sparkly" blouse to formal dresses.
The other 5 nights are Smart Casual. For men, slacks and a collared shirt, for women a sundress, pants or a skirt and a blouse.
They do enforce the "no shorts in the dining room" and "no sleeveless shirts for men" at dinner, but you can generally get away with anything that looks nice.
If you don't want to do Formal night you can go to the Buffet in whatever you want to wear or you can order Room Service.
Have fun and don't stress too much over what to wear