Welcome to the community.
I think your idea is wonderful. There are other stories about witches but they will be different then yours and most never get completed; so what I’m trying to tell you is go for it - if nothing else, your unique writing will make the story interesting.
@FairyGodfeather wrote a thread with links for those just starting. I’ll edit the link in if someone doesn’t ninja me and provide it before I get it.
We all destroy stuff we write. Heck, 100 people can tell me I write well but I always think my stuff is worse then baby-scrawl.
My advice is work on an introduction piece or exploratory chapter - see how that goes and go forward from there. Most demos are between 10,000 words and 20,000, just to give you an idea.
NINJAED! Master List: Links for Beginners