I'm a mom and my then-15 y/o son wanted to get his ears pierced. Yes, he wanted to get both pierced. He wasn't a rebellious teenager - he was respectful towards me, so I agreed. He paid half and I paid half for the piercing.
If you still live at home and your mom has put parameters around this, such as no tattoos, no piercings, etc., then you need to wait until you are living on your own to do what you want.
Other than that, you can tell her that you know she doesn't want you to do this, but it's something that you've thought about and that you want to go ahead with it.
You can be respectful and disagree and the world will not end. I'm glad you are listening to your mother, but at some point, you need to become your own person. Good luck.