First of all, i will tell you my personal idea about hiding your practices or altars from your parents, but since you asked it and you are sure you don't want to be honest with your parents, i will also give you some tips for it, but please consider to listen to what i have to say about being in hide from your own parents...
In my opinion, if you are under the age of 18, you have to understand that your parents have the responsible for you, and sometimes they can make the decisions for you since they are your parents, and as i said they have the right to do that. You should talkto your mother and tell her that you want to practice witch-craft with your heart, be honest to her and make her to see that the craft is really important to you. She is your mother, and i am sure she wants to see you happy, and if you do talk to her with heart to heart conversation, it can be really helpful for the situation you are in currently.
Your mother might be against your practice because she has wrong opinions about the craft, and she might have no idea about what you are doing. So it would be a good try to talk to her and educate her about your practices, and if she was mislead with wrong opinions, try to "open her eyes", and show her she is wrong and that there is nothing wrong with what you do.
Now, this I will try to give you a tip. If your mother is still very sure she does not want you to follow the Wiccan/pagan path, it is her right. Until you are under her roof, she has the right to allow you things, and to forbid others. You said you are 17 years old, and when you will be a 18 years old independent young lady, you will be able to practice what you want, dedicating all you can.
Lets not forget that witch-craft offers you a lot of practices, and not all of them require building an altar, or honoring any specific gods. You can meditate, study about religions (it is good to learn about your religion before you gain experiences, so you can be connected to your path with studies as well), work with energy,herbs, etc. That is actually the wonderful thing about the craft, you will always find something to do which even matches your mother's policies. You have plenty of choices, try to find something good to practice and study about (which your mother will allow), until you will become a grown up. Trust me, it won't be hard to find.
Now, that was my opinion about what you have to do with your mother, but you can of course not choose to listen to what i have said. Now, since the thread is about ways to hide your altar from your parents, i will give youanarticle about it, you are not the only one who asks about this subject, lots of young folks under the age of 18 ask about it. But pleasehave my words under consideration.
I find this article as helpful to your situation, it gives you ways to hide your altar, you could also find other articles about this subject.
Again, have my words under consideration.