It's really simple. Make a contract with me and become a Magical Girl. It's all you've ever wanted isn't it? To use your wish to save someone's life and in return creating a soul gem that will help you fight against the witches?
Your already a witch, one day your soul gem will go black and when that day comes, you will be in so much pain, and then you will become a witch, a witch so powerful that nothing can stop you, except the Goddess that wished to erase all witches before they were even born.
You still want to become a Magical Girl now?