How do i tell my mom i was raped?

So. . . about ten years ago my baby sitter, who was my step-grand-father, would rape me while my mom was at work. i was about 6 or 7 years old.He has passed away since then. This is driving me insane because i feel embarrassed. I don't want to just spring it up on her randomly. No one in my family knows about this, and if i re-call right my cousin was also raped by him. i just don't know how to tell my mom. help??