Write.
No, really, that’s it. It’s kind of a definitional thing. What you have to be in order to be a writer is, you have to be someone who writes. Period. You don’t have to be nice; you don’t have to be educated; you don’t have to follow a bunch of rules; you don’t even have to be particularly professional in your attitudes (though if you aren’t, you can expect to have a much harder time selling your stuff to an editor). You certainly don’t have to be worthy. All you have to do is write.
On the flip side, if you aren’t writing, you aren’t a writer. Talking about writing is not writing. Reading how-to-write books is not writing. Research is not writing. Telling stories is not writing. Writers write. That’s the definition.
Writers write. Which is plenty enough, believe me.
