I would recommend starting with a few recent novels (UTD, Cell, 11/22/63) followed by The Stand and then the Castle Rock books and stories (including Bag of bones). By the time they are done with that, they should be hooked! I mean, those novels were written in four different decades, which I think would not fail to impress anyone!)
SK has a pretty bad reputation among some people thanks to some of the movies, so it would be most important I think, to show them he is so much better than those movies.
I would not recommend starting with Dolores Claiborne or the older novels.