Step 1: Developing Ideas
For a good story you need to have an even better idea. These ideas will change after months and weeks of developing them, so writing them in their early stages could cause issues later in the story if you look back and feel like changing it.
The characters should also be unique, each with their own set of beliefs, opinions, etc. etc. you don't want it to appear as if they all have the same mind. Unless you're writing borg fanfiction, that is.
Now the best characters are the ones that you can relate to, like a character that' s trying to get over a rough patch, or maybe(if you're writing a love story) a character who is deeply in love with another character, but just doesn't know how to tell them how they feel.
Characters should also have realistic ages and do realistic things for their age, because people might get a bad impression about you if your story is about 15-year-old alcoholics...
You might be writing a story about people and then later decide it's easier to tell it through animals, I don't know, but just get creative and get those creative juices flowing! I find listening to music helps me come up with ideas for my stories faster.
Also, there needs to be an interesting plot that will keep the readers wondering what's going to happen next.
You don't want a story that people stop reading near the end because they know what is going to happen. Don't be a predictable writer! Make your story unique!
If you're having a hard time thinking of plots, I found that there are several websites that help with these problems!
Here is one: http://www.creative-writing-now.com/short-story-ideas.html
Step 2: Ways With Words
First off, the first line is what makes people interested, so you need to have a good first line.
Also, if you don't want your readers getting confused, you should also decide if the story is from you, the writer's perspective, or from the main character's perspective. People might get confused also if you write a story from your perspective but use terms like "I", "My", or "Me".
Make sure you also have good grammar, this is especially important because people won't understand your story if you have bad grammar.
A good paragraph should look like this:
"The moon was shining brightly over the forest. Everything had a sort of silvery glow to it... "Dave was right, this place is beautiful..." Sarah gasped under her breath. "It's breathtaking...""
Not like this:
"the moon was shining britely over the forest. everything has a silverly glow on it. dave was right his place is beutiful.. sarah gasped its breathtaking"
Now you should also use descriptive words in your sentences, because you know the story inside and out, but the readers don't.
Words like Beautiful, Disgusting, Hazardous, Strange, Undesirable, Despicable, all adjectives of the sort, basically, are good for describing your story.
Step 3: Balance
A balance between happy and sad moments, especially. Nobody is going to read a story and like it if it's all darkness and sadness, readers want variety! And, plus, chances are, your readers get enough of that darkness and sadness during a normal day - ex. still dealing with the death of a loved one, going through family issues, breakups, etc. etc. etc
I'm not saying your story should be all lipstick, bubblegum and lemonade. I am saying that in a story, there should be a balance between positive and negative moments, because the negative moments will make the story more interesting, but so will the positive moments. But if you have too much positive or negative moments, people might not take your story seriously or they might think it's boring.
Step 4: Get Writing!
What you need is a good program to write on, which you may already have, but if you don't, I'd suggest programs like AbiWord or OpenOffice.
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Another thing I have to say:
This instructable is based off my personal writing experience, so not all of this may be true for YOUR story, but I sure hope it helped and provided you with some ideas and tips.