Depending on your preferred genre, you can technically start anywhere with Stephen King's books. One of King's most popular novels is "It," which was first published in 1986, according to King's website. The story follows a group of people returning to their hometown 28 years later to face up to an alien killer clown who has started killing once again.
Vanity Fair ranks "It" as King's third best novel, whereas The Manual places it fifth, noting that its long length (over 1,000 pages) definitely harmed its score. Nevertheless, a reader poll conducted by Rolling Stone had "It" in second place, with the engaging plot being one of its main positives. Proving its popularity, the novel has had two adaptations, a miniseries in 1990 (via IMDb) and a big-screen adaptation with a sequel (via CBR). Even if you haven't read the book you've probably still heard of Pennywise, the terrifying shapeshifter who gave a lot of people a phobia of clowns.
The general popularity of this book means it's a good place to start. However, be warned that the book, as many of his novels do, does have overtly disturbing content, some of which King has admitted might have been caused by his drug and alcohol addiction at the time (via Daily Mail).
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