I just found out my dad has another family

Yesterday, I went to a friend's house. I had fun with my newly-found friends, even met a guy who was once residing in a foreign country. I talked to him for about an hour and that's when I discovered he's actually looking for his dad who left him and his mother in the States. I asked him who he is referring to, and as a response, I heard my father's name. It was then I learned to realize that I have a sibling, half-brother to be specific. That was probably the most painful truth I've discovered. I just found out that my dad has another family. He didn't tell me he has a family left in America some years ago. I don't know if my mom knows about this, but I need to talk to her and to my dad. How should I approach them? Please tell me how.