I used to hang out in my grandmother's kitchen a lot, possibly because she was usually baking. I was a loquacious child, and often explained to her, in great detail I now realize, what I wanted Life to do differently. Her response was often a terse admonition, "If wishes were horses, beggars would ride."
I didn't get it then, but I do now. Grandma was a practical Scotswoman who knew that you can't just want.
As I present workshops these days for aspiring writers, I see lots of wishers: they'd like to write a book, but they haven't started yet. They wrote part of a book, but they haven't got around to finishing it. Or they've written a book, and they're waiting for someone to ask to read it. All of these wishers are doomed; they'll never get on that horse and ride.
If I had to summarize my advice to would-be writers to one word, it would be "Persist." You have an idea in your head for a great book? Make yourself write it down. You've started a book but stalled? Do whatever it takes to get yourself going again. You've finished the book but don't know what to do next? Research until you do know (take my workshop!). You've tried to find an agent/publisher and failed? Persist.
Everyone could write a book. But not everyone does.