So lets say you've got this idea for a novel. What are the next steps? Well, the first thing to consider is: do you have enough writing experience to actually complete the project? I ask this because even Steven King tells the story of how he put on idea away until he felt he was ready to tackle it. So if you answer my question no, then pull the idea aside and work on other things until you are ready.
But lets say you answer yes. Then you've got a few tasks ahead of you before you start writing. For example, you'll need to know your main characters very well. What makes them unique? What's their background? Their motivation in your story? Who do they fit into the world? Is their action consistent with a person like the one you describe? Do they like each other? If not, why not? If yes, why?
You get the idea. You'll also need the background story so you can relate these points as needed. If you set the story in the modern world, you can probably start writing at this point but watch out for thing you don't know about. For example, if you're main character is a spy for the CIA, you'll need to do some research on the CIA so you can get certain details right. Of course, you'll probably need to make up a few things too because much of that information is classified.
If you set the story in a historic period, you will definitely need to research that period in time. On the other hand, if the story is speculative fiction, you'll need to create the world (or worlds for a space-faring yarn) yourself. This can be as much or more work as writing the novel. So if you answered no to my opening question, a good next step is to fill in the world information you need.
This can include making maps (of cities, regions, the world), designing coats of arms and guild badges, creating new languages, describing and understand different races, like elves and dwarves or vulcans and klingons. You will also want to give through to the use of magic (for the fantasy novel) and the level of science and technology (for the sci-fi book). The absence or presence of psionic abilities or superhuman traits is another thing to consider.
But above all, if you are writing in a specific genre -- you must know it. What conventions are acceptable and what are not. Is the point to confound the reader with a mind-twisting mystery, or make their pulse race with non-stop action? Does the reader expect to be lost in an all-consuming romance or thrilling to swashbuckling feats of your character?
As you can see there is much to do and to know. Be sure you put in the effort into all these things because if you don't, your story will suffer and feel flat. Oh, and don't expect to include any of this in your story. This information is for you so you can make your fiction seem real. Snatches of this information might bubble up into your story, but most of it will never see the light of day.