So here is a video where I show an example closet and show what items I would start with and why, what kind of items I would add next, and where to go from there. The most important thing is to buy items that go together, with either color or theme (gothic, classic, sweet etc).
Buy lots of accessories, some more "showy" but a lot of clothing / jewelry stores have loliable hair pins, bows and leg wear. You can also find really adorable shoes from normal stores. ^^