Shawshank Redemption is his best book, and one of the best movies ever made. But I'd not recommend it as your first book : first, all his other work might feel a letdown after it; secondly, it is not a horror story, and SK is mostly famous for horror (or more precisely, horrifying situations in fantasy settings).
But IT is a bit of a bore, particularly at the end, just like Insomnia (and I am a hard-core fan).
You should start with Cujo, Christine, Carrie or Pet Sematry. Best is if you know the premise, and think - how bad can that be? Eg - you are in a car and a big dog is outside, looking menacing. How bad can it get - it's just a dog, right? Now read Cujo to see SK's take. Or, you get a feeling your car is alive. Do you feel foolish, or do you run for your life (Christine)?
And yes, don't discount the books he wrote as Richard Bachman (to get away from his fame as SK) - The Long Walk, Thinner or the Running Man, could give you nightmares, and neither is about some crazed man running around with a chainsaw (which is what many think is horror). Each of his 5 books as RB are great read.
After you are thru with all that, you could get into his better books like Misery, Dolores Claiborne or Shawshank. I envy you - getting to read all that first time!