If you're really not sure and don't want to break anything, I'd suggest installing some different OSes in virtual machines and try on that first. That might be a learning curve by itself, but you won't take your computer as hostage for your beginner's errors.
There are more user friendly OSes than others. I'd go with a Ubuntu or *buntu flavor just for the fact that there's a lot of beginner friendly websites, tutorials and forums.
My daily PC is an i5-3570k and it's very quick in Kubuntu (that's Ubuntu with KDE as a graphical environment). I think I have "only" 8 GB RAM and it's quite enough for my use.