There is a big trap in just thinking, I’ll write a horror novel. Then picking up and pen and sitting down to write. This will just lead to you forcing something out before it’s ready and most likely ending up half way through with nowhere to go.
So brainstorming and taking notes will definitely stop this happening. Pick your genre; learn the formulas of that genre. Also what a genre will aid you in doing is coming up with a plot, take the Romance plot offered earlier. Make it simple, only add those things that make it intersting. Boy finds girl, but boy likes all girls a little too much and loses girl, but boy realizes that this girl is the girls of his dreams and has a change of heart attitude, he comes up with crazy and even crazier still ways to win her back. Boy gets girl again. Genre fiction is great for custom made plots.
Then create characters. Write down as much about those characters to flesh them out as fully you can. Not only that, have enough characters to keep the story moving as easily as possible. Then think of situations that will develop those people and add life to them in the mind of your reader.
Already you book is taking on form. But we still have setting. Where you story takes place can be very important too. Take Sci-fi. Your where can be anything the imagination can spawn, there can also be a when. Also you there is a wealth of incidental add ons like inventions, such as space ships or time travel. Horror gives you an abundance in monsters and powers of good and evil characters.
We can now create scenes that develop plot. Incorporating all these bits into a coherent whole that move the story in the way you want it to go. If you have enough material in characters, plot and the crucial incidentals like scenery or history or anything else that will flesh out the story coming up with enough scenes to drive the book along shouldn’t be too hard at all.
Good note taking will prove invaluable when it come down to writing because you can easily reference it as you go.