You have an idea. Maybe you have the first three chapters done. But somehow, the rest isn't coming. Is it writer's block? Is it bad plotting? Will it ever become a novel?
Experts everywhere have one answer: butt in chair. You have to write. You have to sit there. You have to keep at it. Will it be good? Maybe, maybe not. But you can't fix it if it isn't there.
Write a whole story. Once you're written "The End", then you can worry about whether it's good or not and begin the process of making it better. What keeps wanna-be writers from becoming writers is Not Writing. An idea in your head is safe and promising, but you'll never know if you've got what it takes until you write it down.
So put your butt in a chair and write. Now. Every day. Until it's done.