well a spell might not be the best idea here. maybe one to give yourself the confidence to speak up, but it's not necessary. wait for the right moment, then sit her down, ask her to hear you out, and explain your choice. tell her why you've decided to cast spells [i'm assuming you haven't converted, you just want to cast spells?] explain there is no devil involved, the spells you do are not to harm others [again, i'm assuming] and explain what a spell is, and how you feel.
i'm sure she won't be happy, but if she says no, don't argue, just accept it, until you turn 18 [or whatever the legal age of consent is in your country] your legal guardian is responsible for you. if this path is right for you, you will return to it.
now, if you've also converted to Wicca, Celtic, Norse, or another Pagan religion, you would like to include more information, such as books, or websites, research some answers to common questions [again, the devil worship question will come up a lot] explain how it makes you happy, how you've chosen this path not to upset her or to rebel, but because you feel complete.
i was in my teens not too long ago, so i know how being told no might cause you to argue, get angry, or to go behind you moms back, but try to do the mature thing. it might seem like a long time, but if she says no, you need to respect your mothers choice. your mother might come around quicker if she sees you've put thought into your choice and haven't jumped on a trend because you watched a movie or something. granted, if she says no, you can still do things like study herbs, astrology, or baking [to name a few] on the surface they seem like hobbies, but they're things a witch will probably need to learn at some point.