The most important part of writing a short story is figuring out why you're writing. Are you entertaining? Informing? Advertising? This will help you pick your topic.
If you are writing nonfiction: make sure you get your research from credible sources!
This is very important to make sure the information you are using is accurate!
If you are writing fiction: decide what genre you want your story to be in. Is it a mystery? Is it a horror? Is it funny, like a comedy? Or sweet, like romance? Decide what works best for you. Then you have to figure out a plot. It often helps to work backwards: start with what happens at the end and work on how your characters got to that point. A clear beginning, middle, and end is also extremely helpful. Throw in some conflict; don't make it too easy for your characters to get what they want. Now that you have your plot,fill in some more details! These are always changeable, so don't think you absolutely have to use exactly what you've written.
Now that you have a plot, you have to figure out the setting. Is it Earth? An abandoned building? A distant planet? If you create a new world, there are some details you may need to include (How is water distributed? What is the atmosphere like? Are there rules or laws?) to make it realistic. (Unless you don't want it to be!)

Finally, you'll need to develop some characters. When writing them down, (perhaps on a separate sheet of paper) be sure to include both physical and personality traits. The more detailed they are, the easier they are to write and relate to. Be sure to include how they get along with other characters! Are they shy? Are they angry? Do they like or dislike certain people? DO they have a crush on anybody? Are there any relationships between characters? Are they human?
Have some fun with them!
If you run out of inspiration for a character, or you can't decide what colour to make their eyes, you can always look at some interesting pictures on Pinterest, Google, or WHI! If you don't know how to move on with your story, trying using a writing prompt (there are tons on Pinterest!) or quotes from famous people. (easy to find on We Heart It!)
So go sharpen your pencils and have a blast! And remember: you don't need to follow this list exactly in order to write well! Do them out of order or add your own ideas! Get creative!