For a server to accessible from the internet, it’s to have a public IP address. Your isp will provider this address. Once this known, you then should be able to access the server by its public address.
Now, you need to purchase your dns name from a valid DNS register. Typically this register will allow you to control the DNS records for this name. You just assign your public up address to the cname record.
https://en.m.wikipedia.org/wiki/Domain_name_registrar
You probably will also need a signed ssl certificate for your DNS name. You will need to purchase this also. You can create the certificate on you server, you will need to pick a valid authority to sign it.