Given the title of your post, I will assume that your mother is a Christian, but without knowing how religious your mother is (or even what particular denomination she participates in), gauging her response for those of us online is going to be difficult.
However you choose to tell your mom of your differing religious beliefs, I do think there are things you should probably prepare yourself for:
- Given your age, there is a strong possibility that your mom won't take you seriously. She might just consider it a phase you're going through. This will probably be very frustrating for you.
- She might be angry with you. I think this is less likely, but it's always in the realm of possibilities.
- If your mom does take you seriously, she is very likely to be upset. You have to remember, to a Christian, turning from God is a matter of your mortal soul. Some of this depends on which denomination your mom practices her Christianity through or even her own personal beliefs (which don't always agree with one's personal church).
If your mom is a practicing Christian, then she's probably attempted to raise you as a Christian as well. Religion and one's relationship with God (or whoever) is a personal thing. Most people I know went through a period in their lives where they were trying out different religions and different beliefs, usually between the ages of 15-20. Most of those people I know were Christian, and the vast majority ultimately returned to Christianity, though usually not the denomination in which they were raised. A few I know converted to different religions altogether. For some it was a true search, for others it was merely a form of rebellion.
Since it seems like you have gone to great lengths up to this point to conceal your exploration of other beliefs from your mom, simply announcing to her that you are rejecting Christianity might come as a bit of a shock to her, and might cause her reaction to be a bit more extreme. It might help if you start the ball rolling by telling her initially that you are exploring other religious ideas and beliefs outside of Christianity, and leave it at that. If she asks for specifics then you can give her specifics, but I would leave the first conversation as simply a way to let her know that you are looking. If, in a few years, you've decided that you are leaving Christianity behind altogether, then your exit from the church won't come as a big shock to her. She may not like it, but she won't be surprised by it either.
On the other hand, in a few years, you may circle back around to Christianity or find a denomination of Christianity that respects the Earth and its creations more than the denomination you've been raised in, and in which you feel more comfortable.
In the meantime, if she attends church on a regular basis then you may be required to go with her. This is between you and her, and the two of you will need to work that out together. I know many parents who live by the "As long as you live under my roof, you will attend church" rule. I know just as many who don't. I stopped attending church when I was 17 and my parents never said a thing about it.
Anyway, I hope some of this helps. It seems from your post that you want to be able to express yourself more freely religiously, but you don't want to sacrifice your relationship with your mom in order to do that. Introducing the idea slowly might put her more at ease.