Set domains and DNS

Hello, first of all, thank you for all the support and sorry for my bad english.

I just installed Ubuntu Server 14.04.1 LTS and installed webmin + virtualmin:

Available features BIND DNS domain, Mail for domain, Apache website, Webalizer reporting, Log file rotation, MySQL database, ProFTPd virtual FTP, Webmin login
Disabled features SSL website, PostgreSQL database, Spam filtering, Virus filtering
Disk quotas Not active on home directories filesystem.
Mail server Postfix

I can create virtual servers, and locally I can access them.

If I buy a domain name (eg how I can configure it to point to my server, and the same with other domains if bought them.

I’ve been looking for my question but I see many different configurations (nameservers, etc) and do not know which one I need.

Thank you in advance.