There’s a quote (attributed to Mark Twain) that encourages writers to “write what you know”. Depending on where you’re at as a writer, this can be intimidating and questions will chase themselves ’round and ’round your mind as you stare at a blank page.
What do I know? How can I make others believe that I know about zero-gravity, if I’m not an astronaut? What if I fail at writing a good story? What if I suck at writing about even the most simplest things I know?
True story – been there, done that. Even now, I still go through this period of self-doubt, but I’ve also developed skills and tools to ignore that Negative Nancy and do the necessary work. Eventually, you will, too.
So, about Mr. Twain’s quote.
My interpretation of ‘write what you know’ is this – start with what you know and build on that.
Take the idea that is circling around and approach it with curiosity.
Let’s say you’ve got this amazing idea about a girl who loves horses. Are you familiar with horses? If so, you’ve got a good start on how to approach your story. Write everything down as it comes to you, keep a running list of thoughts, ideas and questions to come back to for revision.  Some of it will make its way into your story, some will not.
If you’re not familiar with horses, keeping a list is also a good idea. Start by writing down what you think you might know. Then keep a separate list for any questions that come up.
A visit to local riding stable would be in order – there are barn hands, trainers and riding instructors to talk with. Make sure you have an appointment ahead of time, show up prepared with questions and dress in proper attire (this would include jeans and heavy duty, closed-toed shoes), they will be more than happy to assist you.  There may be more than one visit to the barn – especially if you fall in love with a particular equine with soft brown eyes.
Perhaps your character is just as unfamiliar with horses as you are. This would make the story a shared journey between you and your character as both of you learn about horses.  And, by extension, the reader would be going along on this journey of loving and learning what it is to be around horses.
 Keeping an ongoing list is helpful – keeping a story journal is even better.
 This suggestion still applies even if you are familiar with horses – it will give an added depth and texture to your story.
 There are some schools of thought that this is not the wisest route, but as far as I’m concerned, there is no wrong way to write a story.