JThomp42 is correct in that an author should only write about what they know. But if this is too vague, I'd encourage you to look into an instance that you felt a surge of passion. Explore what caused it. What did you end up doing with this moment? What would you have done differently if you could do it again? And place your character in that position. Don't get caught in trying out some grandiose message. Simply tell the world what is moving you heart. Be concise and have your characters make decisions with consequences. First novels are impossible. Think actions instead of subjects. The decisions you make for your protagonist/antagonist will decide this for you. If you complete your story, you will find things that will help answer what your subject is. It is gonna take many drafts and they may slip in and out of your initial subject unintentionally.