How and when do you open about SA/rape to someone you’re seeing?

I’ve never opened up about it and the biggest problem is— HOW do you even start this kind of conversation? Like how do you even say it out loud?? I can’t keep this a secret and I’m just horrified of what kind of reaction I might get because I tried digging online and it’s all terrible..what if he thinks I’m dirty or what if he gets too overwhelmed? Moreover I have no sexual experience (well consensual) so I know this is something I have to talk about but it’s so difficult I’ve never told anyone Irl ever just how do you even get the words out…I want to get it out..it left alot of side effects too regarding my thoughts and some of my behavior and it’s difficult to hide but I just don’t know what to do