To set your store domain, go to the Settings tab and find the Domain settings.
Enter your registered domain name and click Set Domain. Until the process finishes, you will not be able to do anything on the store.
After a few minutes, you'll see the nameservers in the box, and after you've set them and our system registers the change, the final process of SSL certificate installation starts.
If you're not sure how to change the nameservers, here are the help articles for a few popular registrars: