Dress Sizes are just that, numbers that pertain to a certain structure, such as Extra Small (XS), Small (S), Medium (M), Large (L), Extra Large (XL), Extra Extra Large (XXL), and the sizes in between them. These are represented by numbers: 0 to 20 in the United States, 3 to 23 in Japan, 4 to 24 in the United Kingdom, 32 to 52 in France, and 36 to 56 in Italy.
So how do you know which size are you? It is important that you find out what your body measurements are. These are measurements of your Bust, Waist, and Hips. I made a table of Dress Sizes, and their corresponding body measurements (in inches and centimeters.)

Using the table above, you then can find your correct (or mostly approximate) dress size. The dress might be too big in some areas, but you can have that adjusted by an alterations specialist shop.
I hope that helps. Happy shopping!! (^_^)