Ubuntu is the best place to start with Linux for a beginner.
Is server operating system is entirely different from desktop & professional training is required for it?My friends learn redhat when they do the course of hardware networking.
RedHat and CentOS are the same things, they have slow release cycles, which is good for servers. You don't want to be upgrading a server once a month.
Fedora is more of a keep current OS, they have new releases every 1-2 months, which keeps them bleeding edge. You can use CentOS as a desktop OS and Fedora as a Server OS, it's basically up too you.
I don't want to be upgrading my server every few months.