I guess making a best-seller is a two step process:
1. Give people what they want.
2. Tell them about it.
That's the simple part.
But first you have to figure out what they want.
How do you find out?
Well, you can ask them through surveys. You can study other best-sellers in the genre. Or you can give them your texts for free and see what people think about them. (That's the approach the author of 50 Shades took as well as author Scott Siegler.)
And how do you tell them about it?
By having a great cover and a description that makes people want to buy.
And by showing them your book, either because you were a clever author who build a list of subscribers. Or because you use social media. Or you pay for ads.
There you have it.
Oh, and story counts, too. More than grammar and spelling.
I just read this excellent article about it, in fact. It gets boring towards the end, but it's a valuable read: