How to disable the built in basic shop (& add WooCommerce)
Go to Appearance > Editor
Select the Theme Functions (functions.php) file on the right.
Near the top look for this line of code:
add_theme_support( 'post-thumbnails', array( 'post' ) );
REMOVE this line of code
THEN scroll to the bottom and look for this line of code:
REMOVE this line of code, and replace it with these 2 lines of code:
This will now let you install the more advanced WooCommerce or Jigoshop shopping carts.