I'll give you simple answers.
1. Fate/Stay Night's adaption is a horrible one. Don't recommend it if you want the actual story and characterization, otherwise Zero will seem a lot better than it when in fact, FSN is the one with better writting. TL;DR; VN is recommended.
2. Anyways, if you don't want the VN, you are missing out on the Fate route - the starting point of the series - and would be either forced to wait 'till HF movies come out or watch Zero first that isn't recommended.
3. Fate/Stay Night is what you watch first. It's the main story. Zero is an addition that shows you how it lead up to the events of the main story. Call it a flashback of some sorts. You don't need Zero to understand absolutely anything in F/SN and it spoils you on a lot of plot twists in F/SN. Also, there are quite a lot of terms or events in Zero that could either make no sense or give you false ideas without F/SN. Also while Zero is great even without F/SN knowledge, Zero becomes a LOT better with F/SN knowledge as some events in it become a lot more clearer, there are a lot of parallels between Zero and SN characters and Zero is an addition to the thematical discussion and message of F/SN.
TL;DR; Anime is horrible. Terrible adaption of story and characterization. Recommending the VN, but if you don't want to read it, then I guess you could watch the anime. Fate/Stay Night is meant to be read before Zero as both authors agree upon.