You just have to tell them. If they react poorly they obviously aren't good enough for a serious relationship. I've disclosed mine within the first few days of talking to someone, unless we meet in person or call each other beforehand. It doesn't necessarily bring me relief, I'm still nervous to talk to them at the start in case I stutter and it causes my voice to crack or I just really struggle to speak. My stutter is mild but it can vary and get pretty bad depending on the situation and whatnot.