Where do I start watching Fate\stay?

Just watch Fate/Zero first
then Fate Stay Night Unlimited Blade Works (the series NOT the movie)
then Fate Stay Night Heaven’s Feel movies
then you can go back and watch the OG Fate Stay Night 2006 series if you want

from there you can watch any other Fate spinoffs in any order

The main contention is almost always whether to watch Fate/Zero or UBW first. You’re going to be slightly confused regardless of which one you watch first, but things will quickly start making sense after a couple episodes.

The anime adaptation of Zero was made before the anime adaptation of UBW. Zero also occurs chronologically before UBW. But the original source material for Zero was written after UBW. They both “spoil” each other to some degree, which is why there is disagreement