Wear a white blouse and black and dark blue skirt. Length doesn't really matter, but it helps if it is frilly or puffs out. Petticoats underneath are a nice touch, but not necessary. Next, you need an apron and your hair in a bun or braids wrapped around your head if it's long enough, but if not that's OK too. You could add a little cap if you wanted, as well.
Thanks to Lena for this fun idea!