As your first experience, I think Ubuntu is the easiest. Download it here:

https://ubuntu.com/desktop

Spring for a new hard drive, just in case you hate it with the fires of a thousand suns and need to go back. Then you just swap back to your old hard drive.

I can also recommend Kubuntu if the gnome UI of ubuntu seems too phone-like. If using a laptop where addinga 2nd drive may be too difficult, I have just shrunk the windows partition before running the ubuntu installer.