How should you tell your mom your dads having an affair?

This is going to be hard specially for you. I don't think its your job to tell. Why don't you talk to your father and tell him that you know what he was doing. Tell him how you feel, even the pain that your having right now. And then give him something to think about by telling him that he needs to tell your mother personally and if he don't, you will be the one that will tell your mother. Don't let your father fool you or he might even lie to you, just be strong and do what's right.