Belief in a story is the beginning of something great. Don’t know where to start? Then start anywhere.
Will it be worthwhile? Don’t know yet. All you need is belief and the willingness to try.
How about taking the main character for a walk, asking questions and letting them speak?
How about taking the central idea and writing a synopsis?
How about taking the most emotionally charged scene, and rewriting it like a short story?
How about taking the plunge? You don’t have to do it all today… but start. Who knows what comes next? You can decide when you get there. Keep writing, the world needs your story!