Pick a costume that you can dance in gracefully. It should be loose enough for you to move around in and short enough so that your feet don't become entangled.
Choose a theme for your costume like Victorian. Victorian dancers wear long skirts with wide bottoms and ruffles. Your partner can wear dark pants and a shirt in the same theme.
Go Western and dress for the Texas Waltz. Men wear black pants, a black shirt and a black top hat. Women wear long western-style red, pink or white dresses with a western-style ruffle at the bottom and matching high-heeled shoes.
Determine the category you are in before you choose your outfit. Each category may have its own requirements. Research the requirements, often posted online on a dancing competition website.
Check all the rules regarding what you are allowed to wear. It may be different in each country and each age category. Check with your national dance sport association.
Pick something simple for younger competitors like a skirt and plain shirt for girls with flat shoes or small heels. Boys can wear dark pants and a plain shirt with black shoes.
Wear fancy outfits and higher heels if you are an adult woman competitor. Wear black pants and a plain shirt if you are a man. Men may also need a jacket, vest or sweater.