Costume Basics: Long brown hair, long-sleeved shirt, vest, long skirt, poncho, heavy eyebrows (using an eyeliner pencil), red geeky glasses, blue braces, and a necklace with a 'B' on it.
Buy just the accessories kit and any items you need if you already own some of these things, or go with the entire "deluxe" costume below and you'll have everything you need to become Ugly Betty!