some say that because your under 18 you need to honor your parents if they don't want you to practice you shouldn't but there are ways around that. like learning astronomy cooking and mythology
but I too am under 18. im in the broom closet (wiccan term for haven't told anyone I practice.) I keep my Book Of Shadows on a hidden file on my flash drive. a lot of people have a lot of different views on this topic but I think you should do what you fell best but just remember everything could have repercussions
my advice try it for a year, learn the holidays, beliefs, and history not just the magic (if you learning wicca/paganism) then if you like it tell you parents if you find its not for you no harm do not telling anyone.
My mother isn't a Christian (She was raised Catholic but now she's a giant mix of religious beliefs) but here is some tips that I've known to be helpful for anyone who has a family of crazy religious people
1: Books of Shadows are easily stored in a locked folder on your computer
2: Ask to have time to yourself just to destress from you day or anything they'd give you privacy for
3: Magick requires no tools as they're optional for a lot of stuff so work on what you can without stuff
4: Rock is just another word for crystals to those unknowing
5: Lighting candles is common in every religion and the ones in those Jesus containers work just as good
6:If they find herbs or anything of that sort say it's drugs (Do not ever actually say that)
In my experience coming from a religious family for the most part that if you don't treat stuff like a big deal people won't even ask questions
I buy a lot of stuff right in front of some of the more religious members of my family and none of them ever seemed to care and the few who do I explain it's no different than their use of a rosary in Catholcism