When you're writing, especially complicated stories such as Science Fiction and Fantasy, you want to lay out a good concept that your story will be based on.
This includes (but may not be limited to): Setting (location [if a made-up location, name, description, characteristics, major figures, history, government, and other], time period [may or may not be based on our world timeline], age of technology, laws of physics), Species [made-up/mythical species need naming, description, abilities [if any], and where the species is commonly located/affiliated), Characters (names, descriptions, bios, abilities [if any], relations to other characters, affiliation), Factions, History up until current plot, and summary of plotline of story (unless you prefer to let it come out as you write, although even without a basic idea of this you could mess up). If your story takes place in space or on another planet, you probably have more to do in terms of physics, technology, etc.
Then you can write with a clear vision of what's going on.


Aaaaand I'm doing my book's concept right now, hence writing this entry to help clear my mind a bit. >__>''