username and host name

Geee...

I am confused sinc this is my first very very newbie set up in WHM/CPanel.

I understand hostname is a FQDN (Fully Qualified Domain Name). But where does this come from?

Am I suppose to use the domain name that I used to set up as a nameserver?

Ex:

domain name: www.newbarney.com was set up as a nameserver and was given a dn1 and dn2

dn1.yoursite.net
dn2.yoursite.net

Now, for a hostname, what do i use?

admin.newbarney.com

or

admin.yoursite.net


Help!!!!!

My server is up and running but is causing a email and ftp problem since the setup is incorrect. Your advise and help is appreciated.
Thanks in advance.
(ps. if this is in the wrong forum, please move it)