If you want just to add the message before the checkout form you can go to Pages > edit Checkout page and add the content before the checkout shortcode. Some options for the checkout page like returning customer, guest checkout you can enable in WooCommerce > Settings > Accounts & Privacy. If you want to on/off some fields you can use Checkout manager plugin to do this. If you need anything extra then either look for third-party plugin or order additional customization here.
