How do you find your Primary DNS?

It depends on what operating system you're using. On Windows-based systems, opening a command shell window and typing "ipconfig /all" should show you. On Linux systems, "ifconfig" will likely tell you.

If you're setting up the system initially, your IP service provider will need to tell you the primary DNS... OR you can use DHCP, in which case the DHCP Response packets will contain the addresses of the DNS servers your provider wants you to use.