Imagination and persistence are more important than confidence. If you have ideas for a story, go ahead and write a story. If it’s the length that worries you, write some short stories first. To go longer, give yourself permission to write a bad novel. Everyone’s first draft is pretty bad, anyway. If you like the story, rewrite and rewrite until you like the way you told the story. Even if it’s never published, it’s a great experience to develop a world from your imagination and bring some characters to life. Good luck!