As you're just getting started, I would cut your losses and reinstall. To remove the current install, change to the zimbra user (su - zimbra) and issue the command zmcontrol stop. This will stop all the zimbra processes. Then, exit back out to your normal user and remove the entire /opt/zimbra directory. You can then start a clean install. A working FQDN is critical to a functioning Zimbra install. If you can't / don't want to define one via DNS, you should set one up in your /etc/hosts file.