By far, the most common question I get asked by authors working on their first pieces specifically written for publication is: "Where do I start?" Or, they will say something to the effect of:
"I have the idea in my head...I mean I know where I want the story to start, where I want it to end, and the main points of the story I want to include, but I just can't seem to figure out how to start writing..."
Sadly, because each of us has his or her own style and methods, there is no clean-cut single answer that will work for everybody experiencing this problem. However, I will at least supply you with a method that has worked for me numerous times, and which other authors, some bestsellers, have said works for them when they hit a literary wall.
In short... outline. Construction workers and general contractors can't build a house without a blueprint and a floor plan. Some writers can't either. Keep in mind though, just like in construction, there is always the possibility that the project grows and develops in ways that were unexpected during the planning phase. The same is very true when you write. Never lose sight of the fact that your outline is just a guide and isn't carved in stone.
So now you're ready to get started. If you know where you want your story to start, write a sentence or two explaining the opening setting at put it at the top of a clean page. If you know how you want the story to end, write that at the very bottom. If you don't know how you want it to end, that's okay, just skip that part for now. All you really need right now is a starting point, but if you know where you want it to go, it is a big help in crafting the events of the story to lead where you want them to go.
Just a quick side note here; it's sometimes better to use index cards instead of listing ideas for chapters on a single sheet of paper. Index cards will allow you to re-order and interchange the position ideas over and over without erasing or playing a full four quarters of trash can basketball with your draft ideas. If you have a cork board and a few pushpins, so much the better.
Next, try to visualize your protagonists as they move on their journey through your story. For each key experience your character experiences, write another sentence on your page between the beginning and ending (if you listed one) or make a new index card and insert it into the outline where you think it makes the most sense at that moment. Don't worry too much about the order making perfect sense at this point. As long as it makes sense to you for the creative process, that's all that counts right now. No matter what order you choose to place them in at this point, I can tell you from experience, they will almost certainly change later in the actual writing process.