I deadass have no idea what this is, but I've heard that it's good and it has multiple series and what not. Where the hell do I start?
Well in order of airing: Fate/Stay Night is the original. Unlimited Blade Works is next, then Fate/Zero.
Fate/Zero is a prequel then Fate/Stay Night and Unlimited Blade Works run alongside each other like two routes in a VN.
After that jump into Fate/Illya, for some happy fun times and see anime being saved.