First of all you need a really good idea for a book. It could be for a romantic book, or a fantasy book or for anything you want.
A book won’t write itself, it could take under 6 months to a couple of years. Everyone is different and writes at different speeds.
I look at writing prompts to help me think of different scenarios for the characters in my book which then helps me think of what my characters might do next in the current scene that I was writing.
Writing a book is hard, you get writers block or you suddenly get busy. Keep at it. Set yourself deadlines and goals to strive for and achieve.
Don’t feel down if you don’t get it done in 6 months or a year. Everything takes time.