The shop is powered by the "WooCommerce". Check out the full WooCommerce documentation here: http://docs.woothemes.com/documentation/plugins/woocommerce/
Setting WooCommerce Pages (Cart, Checkout, Shop, My Account)
After you import the demo content, you will need to set your Shop, Cart, Checkout and Account pages. You can setup your Shop index under WooCommerce > Settings > Products > Display:
And then set your Cart, Checkout under WooCommerce > Settings > Checkout.
Lastly, set your My Account page under "WooCommerce > Settings > Accounts"
Regenerating image sizes after adjusting settings
After changing these settings you may need to regenerate your thumbnails. You can do this with this plugin: http://wordpress.org/extend/plugins/force-regenerate-thumbnails/
You can manage all of the shop products under the sidebar heading "Products".
Product Attributes (Small, Medium, Large or colors)
You can easily add multiple product attributes like shirt sizes, colors and more. Learn more here: https://docs.woocommerce.com/document/managing-product-taxonomies/
WooCommerce - Options (Columns, Posts Per Page)
You can adjust WooCommerce options under Appearance > Customize > Shop