There are two ways to complete this sentence and you’ve heard both before, I’m sure of it. Somewhere someone sometime gave you the advice to write what you . . .
A) KNOW
B) WANT
They are rather similar even if you don’t think so right away.
Writing what you ‘know’ isn’t writing about a wannabe author trapped in his room all day wishing he could be rich so that all he had to do was wake up and write.
What you ‘know’ isn’t the story. It’s what you bring to the story. Your knowledge of emotions. Your recollections of the details of a scene or facts you researched. That’s what you ‘know.’ All those things you’ve observed other people doing, all those aspects of a summer’s day. Those are the things you know.
And this goes hand-in-hand with writing what you ‘want.’
Writing what you want isn’t just pulling that story out of your imagination. It’s knowing how to pull it out of your imagination. It’s wanting to know how to write it. It’s knowing the details that’ll make your fantasy world feel real to your readers.
You should research, especially if you are writing about a topic that’s not plucked from your imagination. And if it sounds daunting to learn a new subject, then it’s probably not what you want to be writing anyways.
If you want to right it, learning what you need to know will come easily.