On the Server itself launch Terminal and key in this command:
If it comes back with something similar to this:
It already has an fqdn and you don't necessarily need to do anything else . . . however.
OS X Server absolutely requires a properly configured DNS service somewhere regardless of what you're using it for whether it's simple file services such as AFP and/or SMB or a fully fledged network directory server. If OS X Server is the only server on your network then DNS must be configured otherwise the server will behave erratically and not perform as it should. If there is an existing server on your network that is already providing DNS services for it then the relevant DNS records should be or have already been created for OS X Server on it.
To test if this is true find out from your network administrator what the IP address of your existing DNS server is and verify that address is listed first in the Network pane on OS X Server. Then launch Terminal on OS X Server and key in this command:
sudo changeip -checkhostname
Key in the password for OS X Server's admin account (which you won't see being typed) and hopefully (if everything is configured as it should be) it should return - "there's nothing to change". If it returns with something else come back here and hopefully we can take it from there.