Create characters that can experience a transition over the course of the novel. Younger characters are best, as most every adolescent or young person undergoes a life-changing event.
Decide what type of turmoil the character is facing. It can be anything from going off to war, getting married, going to college, facing the death of a relative or overcoming shyness. The event must be something the character needs to face and come to terms with by the end of the novel.
Draw from personal experience. Keep a diary. Use events that have shaped your own life, and inject them into your story. "Write what you know" works best when writing a coming-of-age novel.
Write an outline. Flesh out key points such as what the crisis is, what catalyst sets the story in motion and how the character is going to change over the course of the novel.
Attend a writing workshop or seminar that focuses on writing a coming-of-age novel. Take a college course on novel writing, or enroll in an online writing course. Doing so can help you develop your writing and focus your novel into a coherent story.