How can I tell my mom I have a boyfriend?

So me and my boyfriend have been dating for almost three months, but because it's a secret we've not really met up very much. I think I want to tell my parents now, but as this is my first boyfriend I have no idea how to. She doesn't even know I am good mates with guys, even though he and his best friend are really close to me. Everytime my sisters joke about when I'm going to get a boyfriend, mom changes the subject harshly and dismisses me as too young. I really love him but I don't know how to tell her. I'm her little girl in her eyes, and I'm scared to let her know I'm growing up. What do I do? How should I tell her? Or should I keep it a secret? Any help would be greatly appreciated, as I need to make a decision fast. =/