Want to know what it is?
Here you go:
Read a lot
Write a lot
That's it. So many times, I think if the people asking the questions would do these two simple things, many of those would be answered. They want to complicate it and make it seem more mysterious than what it really is.
Many of our problems with writing can be solved this way: read a lot; write a lot.
The second line (write a lot) however must have one more element to drive you forward. It's passion. I know there is a lot of talk lately about being passionate about this or that, but it's true. If you truly loved writing--and I mean so much so that you deeply yearn to sit down at your computer or tablet or whatever you use to write on--you'll find a way to make it happen.
What are you waiting for? Do a little more of the latter than the former, but by God do both. Many of the answers are right there.