Being a witch isn't a physical transformation it is a spiritual one in which you choose to practice a dedicated path of which their are many depending on your definition of Witchcraft. Among witchcraft there are many cultural, ethnocentric, and personal types of witchcraft you may wish to study and explore. If you find any of these suitable to your spiritual and personal idea then you can begin to dedicate yourself to that path and start following the beliefs, traditions, rituals, ideologies, and other said practices of that specific path that you have chosen.
A good place to start is by reading up on the different types of witchcraft there are, or other spiritual and magical paths. You can do this by reading posts here in the forums, by searching google, reading articles and websites, and by reading books about such paths. From there you can begin to narrow which paths interest you the most and give those paths further study until you find what's right for you. Just as well from my own personal experience my spiritual and magical path as well as identity has changed many times and I find rather fluid as you learn, experience, and grow.
Good luck finding your path.