That depends on what you mean by the term "I."
If you mean your physical, flesh-and-blood body, it has developed over time by biological and genetical processes. I suggest you enroll in a Biology and Genetics course in your school or in a local College or University and learn more about it.
If you mean your self-aware ego, or Consciousness, that is a Philosophical, Ethical and religious question. I suggest you enroll in a Philosophy course in your school or in a local College or University and learn more about that.
In my own, personal opinion, "I" is the unique combination between my physical body and my conscious mind, which were united by God when I was born. Other people have other opinions and definitions.