Nothing happens. You don't even exist, yet. How could you exist before a specific egg from your mother, and a specific sperm from your father, amongst millions of different eggs and sperm, happen to get together and begin the process of creating you? That's where your "start" begins, not some rendition of Souls-R-Us, whereby there is some soul that is you, just hanging around, waiting, to "possess" this newly created life begun by the getting together of a certain distinctive egg and a certain distinctive sperm.