Recently a friend of mine asked me for advice on writing & eager to help I asked if there was anything in particular she wanted insight on. She asked me how I turn an idea into a story. Wow. What a question! I wasn't sure what to tell her at first, because it was such a vague question--and a big one too! Eventually I told her a few key things & because she had to leave I promised I would blog about them to go into further detail, so here goes.
1) Ask yourself "why this story?" Why do you think this particular tale needs to be written?/Why do you want to write it? What's the point? This piece of information is the most important because its what you'll form your entire story around. You could write a good story & it woudln't be worht a snowflake in the Artic without a reason or main point. This is the hardest step in my opinion, but once you figure this out everything else will be easier. Which is why you should always do this first.
2) Okay, now you've got a point, what are you going to do with it? You need to figure out your plot, or main storyline. Every writer will tell you something different about how to go about this. Some say the best way to go about writing is to start from the beginning and continue in chronilogical order, building off each event. Others say to start from the end and work your way backwards. Robin Parrish gave great advice here* when he said to figure out the ending & key points and then play connect the dots. It really depends on the writer & the story what works best, but whatever you end up doing, always have a battle plan. If you don't, you'll charge headfirst into a war against a clan of highly trained needle ninja only to find that you're wearing balloon armor. It's not gonna go well.
So figure out where you want to go, and where you're going to start. It doesn't matter what you write first, you could write the very middle first, but know your general road map first. The plot will develop as you go along & maybe the end or beginning or something will change. As facts stand though, this is generally a good thing, so let them change. But don't try to start off without a general idea of how your story will go.
3) Characters are essential, I cannot stress enough how important characters are. No story was ever written without a character, even documentaries focus on specific animals in a group. Sometimes the characters come to you first and are the reason you get the idea for the story, sometimes you have to fit them into the story, but they always come because they need to. Make sure you know your characters before you start off--more than just their name** I'm not saying you need a full biography on each character, but at least get to know them a little. Just know who they are & why they're there; they will tell you the rest in time (and more). Every character is a story unto themselves, always remember that.
When you have a nice cast of characters, figure out how they'll interact with each other, this will help a ton in the long run.
4) Figure out your setting so the reader doesn't have to. If you leave out all indicators of setting the reader might think your Neptune space epic is in Oklahoma. Again, specifics aren't necessary at the beginning, but the basics are. As is (always) the "why" of it. Why Neptune & not Saturn? Why an abandoned cottage & not an overbooked 5-star hotel? Where are you going to make your characters live?
From there, the rest is up to you. I wish you all the luck in the world (or however much I'm allowed access to) on your bold, daring, and dangerous endevour of writing a novel. Some people can't handle it, some writers never make it, but maybe someday, you will. Writing is not for the faint of heart, at least not real writing; anyone can write a book, but it takes a true artist to write a good story.
* Max Hsu's interview with Robin Parrish
** Sometimes, as the story moves along, the character's name might need changing. Originally I had a character named Ulrich, who became Matt. I think this change was one of the best things I've done for the character.
Note: I am not an expert. I think Justine Larbalestier says it best.