Which Linux to use?

I am a rookie in Linux.I heard a lot about Linux OS but I haven’t really touch on it yet.I am planning to install Linux as one of my operating system and I went for research in the market.I find out that there is various Linux OS or utilities in themarket and I don’t know which one to choose for my replacement Windows 98 operating system or a operating system that can function as a file server for me.
There is Linux Red Hat,Linux sendmail,Linux that stand on top of Windows platform,Unix version
and more….
Please advice on the version also.
Thank you.