By default your site will allow guest checkouts. You can change this and only allow registered users to checkout and/or allow users to register before they checkout. You can change this setting on a per Checkout Flow basis, so you can allow guest checkouts for some flows and not others as you need.
Visit the Commerce configuration page and go to the Checkout Flows section.
Click Edit on the Checkout Flow you want to configure.
To Enable Guest Registration
To Disable Guest Registration
Your first checkout page will then have the below sections, its exact appearance will vary as the below image uses the Bootstrap theme.
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