You should be totally honest, man.
Take her to a quiet place. A place where she can get mad at you (she'll be shocked and say things like "Why didnt you tell me before?") Well, this is totally fine. Any person is going to freak-out at first at such news, so it OK. If I were you, I wouldn't directly start the conversation with something like "Look, I have a child". I'd suggest, first tell her what got you into the situation and end it with you telling her the main problem that is the child part. I'm telling you this b'cuz some girls just runs away after hearing the worst part and doesnt want to listen to the rest of the part anymore.
Now, on your part, try not to force her to stay (continue her relationship with you). If she understands you and stays then its really cool (She's really into you). But if she decides to break-up with you then you should just let her go. Thats b'cuz even if she stayed without her own will to stay, she's obviously going to regret it in the near future and could cause problem in your life (esp for you child).
And one more thing, give her time to decide. Its going to be a tough decission. Even if she's mad at you at that present moment, she might cool down later and come to understand you. Give her a day or two.
Good Luck bro.
And congrats you're gonna be a father.
