Coming out ideas

Hello, I'm bi and I need a way to come out to my mom. I just need someone in my immediate family to know especially now that I have a girlfriend. I felt that my mom was the one Person that would understand the most. I don't want to be outcasted by my mom because of this. And I think that she will brush it off because I'm still young and she might think that I'm to young to make these decisions. I'm ready to come out though. Sorry for rambling I just really need some ideas.