My background is in marketing so I could probably write an entire book about how to market a book. There is so much to say. To keep it simple, I'll say a few things about this and then move on.
First, writing is not a meritocracy in the same way as professional sports. If you are the champion in your league, that means you have tested your skills against other top talent and you have risen to the top. If you are not a champion, you can often look at the champion and see what areas you lack. There is a chance through hard work, natural talent and ability, a little luck, proper coaching, the right equipment, etc. you could one day be champion too.
Writing is not like that. The best books don't become bestsellers, and conversely, the bestsellers aren't the best books.
Selling a book is a lot like selling anything — toothpaste, an energy drink, a bag of chips... you try to do the best job you can with the actual product, make it better than the competitors, ensure the packaging is enticing, set a competitive price, try to get it into stores and be places in a desirable location where your product can be seen by customers and hopefully be bought.
If you have the budget, you would also run advertising to promote your product and get in front of as many consumers as possible.
My point is, writing a bestseller has a lot to do with discoverability than it does writing. It often starts with good writing, but a huge component is promoting it to the right people. This begs the obvious questions: who are the right people, and what is the best way to get in front of them?
Once you feel your manuscript is ready, the next thing you should do is make it available to be discovered.
You can do this by creating a website, promoting through social media platforms, including paid promoting, start a blog, be a guest on someone else's blog, make a book trailer and other videos, post on Wattpad, upload to all the major ebook sites (made easy through Smashwords), and have people to review it, especially influences who can amplify to a large shared audience.
It takes a lot of work, but eventually, if your writing is good, you are active on social, and you do a number of other things, you'll eventually start to build an audience.