I got into King a few years ago when I saw Dark Tower VII on sale for $5. I thought, at the time, that it was all seven 'chapters' in one huge book, and bought it. Later realizing my mistake, I started picking up the first six chapters, and have been hooked ever since. I thought Wind Through The Keyhole was a nice little addition to the DT series.
Cell was fantastic, as were Duma Key and Lisey's Story. I loved Under The Dome (until the abysmal ending) and 11/22/63 is one of the best books I've read in years.
I've just recently started going back through more of his older stuff, and am loving all of it. I just finished Pet Semetary (really good, but not as scary as I'd been lead to believe) and am halfway through The Shining. The Stand was epic...that's really the only way to describe it.
I believe reading the DT series is the best way to start if you've not read anything of his. As almost every other novel ties into it, or in some way mentions or connects to it, the DT has made reading his other novels that much more enjoyable.