Publishing your site:
To publish your Website Builder site, simply hit the Publish button, which is found on the top drop-down menu.
You'll then be presented with this message:
Hit Publish again and your site will be live! You'll be shown this message, from which you can visit your live site.
Taking your site offline ("unpublishing")
To take your site offline again at any point, simply go to Manage -> Site Status and select the Offline option in the Site Visibility drop down. From there, hit Update Site and your site will be removed from the internet.
To revert this change and make your site visible online again, simply switch the Site Visibility back to Online and hit Update Site once more.