My only suggestion would be: Browse the forums here, especially the distro forum. This sort of question is asked all the time and answered too.
What you want to do is ask yourself, what are you going to do with your system will help determine what distro to get. Even though, all of them can pretty much do what the other one does, some come with easier config tools, or some make you hand edit files to get your system going. Do you want to learn the OS or just want it to work with ease would be a better question ?
Also, if you never used *nix systems, your gonna want to browse the web for docs on it, commands.. etc.
Good place to start is here, www.tldp.org
for documentation.. and so many more, just by searching.
Another piece of advice is, don't settle on just one distro, try several like Redhat, Mandrake, SuSE, Slackware... or even Debian... you won't know which one you really like til you try them out.