Well, to be honest there is not much you can do except be there for him. Other factors go into this as well such as if it's treatable, how much time he has, etc. As for how you should feel. . . Well, if you are on good terms with your father I suppose you should be sad. How do you feel currently? I mean, you could be in a state of denial and just feeling numb to the whole situation. Just take a breather and analyze the entire situation before going overboard and driving yourself insane.