Actually, the first thing you need is an idea. Sometimes it's visual, and sometimes it's a though that's begging for words.
We have ideas all day long. We also see things that pique our curiosity. It's a good idea to take out your phone or camera and snap the photo - and mark it as a something you may want to write about later. Some with thoughts - if you have a good one, make a note on a piece of paper or your phone to write about it later. It's also a good idea to write down what was happening when you had the inspiration, and how you were feeling, and where you were at the time. Often, our surroundings influence our ideas and give us inspiration to develop them.
Once I was playing with my cat - he was trying to catch some flying string. He was so having so much fun, I got my camera out and snapped some shots of him with his paws in the air.
One of them looked like he was taking a selfie. You know...not perfectly centered and an out-stretched arm in the frame. And lots of fur. It made me laugh, and an idea was born.
I wrote a short story about how difficult it is for a cat to take a selfie because he has too much toe hair to press the shutter button. It doesn't matter that cats don't really take selfies, the photo practically wrote the story itself - it just had to be told. I'm glad I listened.