That's a very good question. I doubt that it can be answered in a way that will make anybody a best-selling author right off the bat. There's more to it than meets the eye, or eyes as is the case.
Oh, there are many books out there on the subject that give tips on writing a bestseller ... do an Internet search and you'll see what I'm talking about. I have quite a few of those books myself, and I do read them and I do absorb them. I even apply some of the tips to my own writing.
But as I've said before and I'll say it again, first off, you must be a reader, you must be an avid reader to even begin to write a novel. You must learn how to write smoothly and succinctly and the best way to do that is by reading successful writers in the genre of your choice. You'll learn the rhythm and the pattern of using short and long sentences, how dialogue is written. You'll learn when an author uses too much description or not enough to make your readers feel, hear, see, and smell what's on the written page. Pay attention, it's all there for you to grasp when you read other authors. Read authors who aren't quite as accomplished, too. You'll notice the difference in the writing. So yes, the best way to learn to write is to read.
The second best is attending writers conference workshops, writing classes in your community colleges, online classes, all of it adds to what you're going to need in your writer's toolbox. Reading how-to books, grammar books, punctuation books ... there are some really interesting ones out there that make it not so text-bookish, you might even enjoy the challenge. I dreaded my English and writing classes in high school, I didn't like them at all. So boring to me they were, and I only skimmed the books just enough to write the assigned book reports. I was a terrible English student. But now, I'm over the top about it, about reading and writing. I'm captive of English grammar, punctuation, spelling, and so forth. It's become my life as a matter of fact. Amazing that it's turned out this way.
So ... while you're doing all this study and self-educating, start writing your novel. Don't wait. You don't have to have it all laid out, or outlined, or perfect, just start writing. You do need to know who your main character is, however, and what his/her backstory is: parents, upbringing, beliefs, personality type, dreams ... you might even write your main character's bio on a couple sheets of paper to have handy as you write, to keep you on track as your character weaves through the plot. That's what I do, as well as a list of the other characters as they appear and are developed.
But how do you know that what you're going to write will sell? You don't, unless you're Nora Roberts or Tom Clancy or Ken Follett and the list goes on. How do you write a novel that sells?
Well, you don't want to be a copy-cat, but you know whose stories you like the best and if you say you want to write like that particular author, terrific! You've found your genre. Write the genre you love to read. If you've read enough of your favorite author's books it will automatically rub off on you and become second nature as you write. The patterns of speech, the rhythm, etc. So that's good. And if you read several authors, you will have a combination from each of them in your head. But you need to cultivate your own voice, your own stories now. That will come, the more you write.
In classes and how-to reference books you're taught the tricks of the trade, how to hone your story, create your characters and plots, all to help make your first novel a page-turner and one that everybody will want to read. Next comes the publisher.
After you hire a professional editor to help you polish your work, you can either go the route of self-publishing or submit to an agent who will pitch it to a publisher.
So, if you've done your homework and your characters and plot bring something new and interesting to the mix, if you have writing talent and you know you've done your best, and your manuscript is polished and professional ... that's all you can do at that point. You've done your job for now. The agent and publisher will take it from there, they will decide if they want to take a chance on it, and whether or not it will sell.
So in the meantime, start writing your next novel. Whatever happens, keep writing.
AND ... keep reading.
Good luck in selling your first novel!