As you progress in the game and gain points you'll visit the Boutique to trade points for new clothes and accessories. Once items are "purchased" - remember you're not actually spending real money - they will appear in your Closet and you can wear them whenever you want! Remember, the cost of an item subtracts points from the score you've worked hard to earn, so be sure to get what you want. You may have to save points for a while so you can buy your favorite item or outfit. While in the Boutique you can still chat with your friends and see what's going on in the game.
When you see your Avatar for the first time, heads up - she's green! She'll be wearing jeans, a t-shirt and flip flops. You'll be able to change a couple of things right now, but now much. You'll find options for the available items along the top of the page in your "Closet". Everything else you'll have to wait to purchase once you've earned some points!