WooCommerce is only a plugin and won't change the appearance of your site. You won't see any changes until you go to a product or checkout related page.
If you'd like to try a free WooCommerce enabled theme, there's a list here that can get you started: http://www.woothemes.com/2013/09/5-free-woocommerce-enabled-wordpress-themes-to-kickstart-your-business-online/
This article would also be a good resource to get you started: http://code.tutsplus.com/tutorials/getting-started-with-woocommerce-initial-setup--wp-29011