This one's a huge catch-all. The knowledge we need to write. On one level, of course, all you need is to write. (And if you don't write, no matter what you know, you aren't a writer. See this brilliant summary by the ever-clever and profane Chuck Wendig). So you need enough self-knowledge to know you want to write, and enough self-discipline to sit down and do it.
But it helps if you have some other kinds of knowledge: the mechanics of writing, grammar, proofing. These things are pretty self-evident, and can be studied if you are weak in any of them.
And then there is the knowledge that I didn't even know I needed when I started this journey: knowledge of publishing and marketing and book design. . . If you are starting out as an author-publisher, do better than I did and take time now to start learning about all of these. Join the communities at Goodreads.com. Start reading blogs (there are about a million that deal with all aspects of writing and publishing). Start your own blog, do the A to Z Challenge, be a part of communities.
|This is what I got when I did it myself|
|And this is what I got when I paid someone to do it right.|