"What do I have to do to become a witch?"
Well,as you are only 13, it will be obvious that you haven't studied for more than a few years. Now, you say your Uncle is a witch. So the question you now have to ask, is, "What is a witch?"
A witch is a man or woman who practices witchcraft.
But, here is the problem; a great many people call themselves witches who do not practice witchcraft at all! For instance, followers of Wicca, often call themselves witches; when they should really be called Wiccans. Because Wicca is a religion; witchcraft is not!
You may think, for instance, that witches cast spells? Some do; many do not! I have never cast a spell in my life.
So, the next question is, "What is a spell?"
A spell is a ritual attempt to obtain a magical result.
So, now, girl, you have to study more.
But don't believe everything you read on this site!