Telling bf about past

Hey! Um so I don’t know how to start this off but please give me some advice. I’m a junior in high school. When I was in the 7th grade my best friend, molly, sexually assaulted me at her house. ive told my parents about it last year and they didn’t help.. I feel like I should tell my boyfriend about it since we plan on doing sexual stuff in the future... I still have issues with myself because of the assault, I guess you could ptsd?.. I need advice on how I should tell him without freaking him out. I need advice on basically everything with this situation. Where should I? When should I tell him? How should I tell him? Please help me.