Dream on kid. Mothers find out everything sooner or later. But it's amazing how little some folks know about their mums. You might as well talk to her about it if it means that much to you. But before you get a tatt, have a look at some really old people's tatts and think about if you'll like to look like them when you're old. They are permanent!
You cannot really hide a tattoo forever, so I would suggest talking to your mom about it. If you are under 20 years of age, I would suggest thinking about this through because a tattoo is not a necklace that you can just remove or take off when you want.
If you are 18 and you are using your own money, your mother really doesn't get to vote. If you aren't 18, I don't know of anywhere you can get a tattoo without a parent's permission. At least not by anyone who doesn't want to risk a lawsuit from your mother.
Why do you want to do something that you have to go to so much trouble to hide? Wait awhile. Think it through. You can always get a tattoo later. Once you get one, it's a lot harder to change your mind.
You don't deserve a tat if you have to get it dishonestly. If you aren't man enough or woman enough to have integrity with your parents then you aren't worthy of one. I'm tatted and I went with my son for his very first tat.
You should really just talk to her about it and if she does go mad about it, you can tell her it's your skin and not her that your "ruining" and that the subject of your tattoo is so important to you that you feel safe marking it on your skin forever.
Forget the tattoo - get a fake tattoo - you will get sick of it eventually and wash it off. Don't go along with the crowd and get a tattoo just because everyone is doing it. Once you lose your parent's trust, you will not get it back 100%.