I'm gonna go against the grain and say maybe don't start with King at all if you're a seasoned horror fan. He's foundational, but his voice is very specific. Try something by Clive Barker or Shirley Jackson first, then come to King to see where a lot of modern horror tropes got polished. Sometimes context helps you appreciate him more.
Carrie' is a solid, concise starting point. It’s his first novel, so it’s where his voice began, and it’s mercifully short. The epistolary style with news reports and articles makes it feel immediate and real.
You get the classic King themes of bullying, cruelty, and explosive revenge, all wrapped up in a tight narrative. It’s a quick read that delivers a huge emotional punch, showing his knack for making you sympathize with someone who does terrible things.