How do I tell my mom about a bad grade?

I have a C in one of my classes, and that means I won’t be on the honor roll this semester. But, that also means I won’t get an academic letter, which is very important to my mom. She’s going to find out sooner or later, but I don’t know if I should wait until report cards come out and she finds out herself, or if I should tell her myself. And if so, how?