Writing instructors often tell their students to write what they know. That's good advice, but it's also important to understand the audience you are writing for. What do you know about them? What do they like? Understanding who you want to reach will make you a more confident writer and help you connect with your readers.
Once you select your audience, read as many books as you can that are written for that particular group. Reading the work of good authors will make you a better writer, because it will challenge and inspire you. It is the best way to study the craft of writing.
Every story needs a clear theme, which is what you are trying to convey to the reader. It is sometimes referred to as the moral of the story. For example, in the fairytale Cinderella, it's not the evil stepsisters who win in the end. Cinderella, the good girl, ends up with the prince. The theme of that story is good wins over evil. Short stories and children's books usually stick to one theme, while complex novels may have several.
If you're passionate about your idea, you will turn out much better story than if you just felt okay about it. If you're not passionate about your story, no one else will be, either. Writing comes easier if you choose topics that are important to you, because you will be able to convey how you feel and evoke emotion in your readers.
This concept may be one of the most difficult to grasp in the writing process. When you show something in a story, you allow the reader to experience it as if he or she was the character, whereas just telling the reader something can get boring. An easy way to show and not tell is to use dialogue so the reader hears the character instead of the narrator. You can also use sensory words to describe what the character hears, tastes and smells.
Many new writers tend to overwrite while looking for those perfect words or phrases to describe every situation in a story. A good rule to follow is to keep your writing tight and simple, and make every word count. There's no room for blather in shorter works of fiction, so if it doesn't add something to the story, delete it.