He's written so many books that it could be a little intimidating to pick up your first one! I have some handpicked suggestions to pique your interest.
What most fans think are his best books
Start with The Stand, which is an epic plague novel told from many characters' perspectives, that ends with rebuilding society and a good vs. evil showdown. But emphasis on the "epic" -- if you're not up for a lonnnnnggg read, skip it and come back once you're a Constant Reader. Instead, or in addition, try 'Salem's Lot, arguably the scariest vampire novel ever written, or It, if you're not afraid of clowns (good luck staying that way).
There's also The Shining, if you're looking for a ghost story, or if you never got around to reading the book that the Kubrick classic was loosely based on. It can be read two different ways: straight up, as a haunted hotel that possesses the caretaker wintering alone with his young family, or, as Ursula K. Le Guin, opens a new window would say, on a slant: as an isolated alcoholic's descent to rock bottom, fueling his resentments with supernatural lore.
If you're intimidated by the sheer heft (and drawn-out endings) of most of his books, try The Girl Who Loved Tom Gordon, a slim, tense story of a girl lost in the woods, or try any of his short stories and novellas: Night Shift is my personal favorite, and The Bazaar of Bad Dreams is a newer collection. You can even listen to the story The Shawshank Redemption movie was based on in just 4 hours (that's a trip to Phoenix and back, not that I know from personal experience, ahem).
Best older classics
Misery imagines a rabid fan who decides to capture her favorite author (in an extended, uncomfortable metaphor for drug addiction). Pet Sematary is arguably his scariest and most unpleasant work. Or go for the best of his "killer ____ with super powers" books: Cujo is the one about the dog. Christine is the one about the car, and Carrie is the one about the outcast high school student.
Best newer books
Mr. Mercedes is a quick read inspired by the tragic headlines about drivers who plow into crowds of people. The Outsider is as scary as any vintage King. 11/22/63 is a pretty amazing time-travel book about trying to prevent JFK's assassination. Full Dark, No Stars collects some chilling stories and novellas that will stay with you.
If you're not too into horror (although, why would you be reading this blog post if you aren't?!), try On Writing, a slim volume that combines memoir with writing advice. The Eyes of the Dragon, an excellent high fantasy story that he wrote for his children, or start his epic fantasy series with The Gunslinger. Although his Dark Tower series is one of his most popular works, I omitted it from the "fan favorites" section above because so many people have trouble getting into it, and the quality's very uneven. (To be fair, I doubt many could write thousands of pages set in the same universe that were *more* even!) If you enjoy his unique blend of fantasy and western, and can get past some of the dated stereotypes and the kind of cold, impersonal beginning (it picks up in the second book, I promise!), it's a long, satisfying read, especially after you've read some of his other books (since they all echo into the same universe, in the end).
Also, pro tip: google "where to start with Stephen King," and there isn't exactly a dearth of opinions out there. 🙂 Here's my colleague Toby's takes on his 13 favorites!
Note: many of these are also available as audiobooks, and there's a reason they're so popular in that format! His pacing, dialogue, and creepy subject matter make for engrossing listens even on the most routine commute. We wouldn't get patrons checking out giant 20-odd CD behemoths if they weren't enjoying them, right? (Don't do that. Download it. Wayyyy less heavy!)