So you could come up with a backstory that you were a Dunmer coming to Skyrim for a better life but became disgusted with how poorly you and your kin were treated. At least, they aren't perceived that way. They're skilled with magic, are somewhat sneaky, and aren't the most upstanding citizens in Skyrim. Something about Dark Elves make them seem like the natural fit. Without further ado, here are my recommendations: Race - Dunmer (Dark Elves) or Breton This isn't a min/max build, but instead meant to help you look, act, and feel like a necromancer.
Keep in mind too that I'm recommending things from the standpoint of roleplaying. This is, in my opinion, the best way to play a necromancer in Skyrim Special Edition without throwing in the crazy customization that mods bring. That's why I wanted to put together a short guide to help you out. For example, if you wear heavy armor, use a two-handed weapon, and raise the dead, you're more of a death knight than a necro. but when you stray too much, you aren't really playing a necromancer anymore.