My father just passed away in PA. How do I contest his will?

It depends.  It depends on what his will says.  Your father may have intentionally omitted  his natural children and if such was intentional he should have stated it with specificity in the language of his will.  If there was no intentional omission.....In the state I practice (IL), the probate statute provides that within 6 months after the admission to probate of a will, any interested person (herein you & your siblings) may file a petition in the probate proceeding in the court in which the will was admitted to contest the validity of the will.  You'll have to give notice (meaning you'll mail or deliver a copy of the filed petition) to the representative/executor or his or her attorney of record, and to each heir and legatee whose name is listed in the petition to admit the will to probate and in any amended petition.  The representative/executor has a duty to defend a proceeding to contest the validity of the will.  You will have to have grounds to contest the will. In this case the grounds may be undue influence, fraud or mistake. You may demand a trial by jury or the judge can conduct a bench trial.  That should be enough information to get you started.