The first person who replied hit the nail on the head, but I would like to add to what was said.
"Witch" is a title you give yourself after you have been practicing withcraft (Just like someone who draws might call themselves an artist). There is no need to cast a spell to become a witch. All you need to do is start practicing magic of some kind.
Although, I would suggest you get a grip on what magic actually is and what it can do before you dive headfirst into any practice. there are quite a few articles and forum posts on site that will probably be helpful in that. Go to General Info in the forums and read the pinned threads.
I wish you the best.