If your site is open to the public and you want a user to register first before they can purchase a subscription to your site, you can achieve this by following the steps below via WooCommerce settings.  

    1. Navigate the WooCommerce dashboard - Settings - Accounts & Privacy. 

    2. Under Account creation, you can tick on the option "Allow customers to create an account during checkout". You may refer to this screenshot:

    3. Don't forget to click on "Save Changes" below the page to apply the setting.