To Moms with teenage daughters

What would you do if you found out your seemingly happy teenage daughter was suicidal and self harming? 
Please answer honestly, I'm a teenage girl considering telling my mom and I want to know how she'll act. I have a few questions.
How would you react? How would you feel about it? How would you attempt to help? In which way would you be most comfortable with her telling you? What would you do? Who would you tell?
I'm talking about a girl that seems silly, happy, and you'd never think she'd even consider this. And you find out she's been doing it for around a month. 
Please be 100% honest. If you all say you act calmly that's how I'll expect my mom to act, and if she freaks out, I don't know what I'd do. 
Thank you!