That may seem too damned obvious for words, but a great deal goes into making any book well-written: spelling and grammar, character, plot and structure, your own reading experience, and the bottom line: some writing friends.
Nothing puts off a potential reader more than poor spelling or grammar, and that will be it: your book won't be read or reviewed or sold.
So if you write in MS Word or LibreOffice, both have a spell check function: use it often. Grammar is harder, but if you're not certain of a structure, a simple online search for something like "should vs shall" will give you a range of opinions and references.
Check them out and see what fits your own work, especially if it's set in a different time or place with different modes of expression. That's also essential for colloquial speech: the slang of even five years ago is different from that of today.
It's well worth the money to pay for professional editors to check the manuscript, as they will see problems your eyes simply cannot see after working on it for a long time. Most of the channels on the Pathways page have contact details for freelance editors.
Today there are many brilliant resources available to help you with writing elements such as character, plot, style, structure: the guts of your book. Use community courses, books, articles and online resources till your head is spinning, then wait a while and do other things. Come back a few days later and see what's settled in your mind, then try again.
Spend a few minutes over your first cup of tea in the morning and just muse on your people, your world. Does the structure meander like a river or rise and fall like craggy mountains? It doesn't matter if that is how you want it. If not, rethink it. Where are the weak points? Where are the pivots? Then rewrite.
Find out what the latest plot cliches and stock characters are and avoid them. But use that knowledge of cliches too: can you find some new words, some new situations, some new denouments? You won't be able to do so unless you know what's already been done (boringly) to death.
Find out what touches you as a reader: can you recreate that response in someone else? Identify it: what was your response? How did the author do it? It might have been a single poignant word, or an implacable buildup over pages. Re-read until you see the how the magic is done, and how you might do it yourself with a twist, another era, a different viewpoint.
Aim to get together every few weeks in a quiet place for two hours or so and email each other beforehand with a chapter or two of manuscript. Be prepared to put in a bit of time reading and gently critiquing other people's work; be prepared to accept different views on your own – they'll hurt at the time and make so much good sense later.
You writing group will help you see typos, gaping plot holes, stretches of logic, absurd cliches (he dropped his eyes to the floor). They'll understand and support your struggles, just as you will theirs.
At worst you'll meet people you might never have otherwise, and at best you'll make a group of real friends who understand you and who'll help you create the finest book you possibly can.
Next step is understanding the routes to epubbing – all those sites clamouring to be the ones to help you: for free or a little or a lot of money, in format A or format B or format X, with a cover image of so many pixels, for specific vendors or devices, or for a bunch at a time, or for option after option. Go to 3. Pathways.