What is the ruling on marrying a Christian girl when there was a previous relationship based on fornication, what must both parties do if the marriage is permissible, and what is the legal punishment for repeated acts of fornication?

You must initiate sincere repentance and sever your relationship with the girl; it is not permissible for a Muslim to be in a relationship with a woman who is a stranger to him. The implementation of legal punishments (hudud) is the prerogative of the Muslim ruler, and a Muslim is not obliged to confess to a crime that incurs a hadd punishment. Rather, he should conceal his sin and repent between himself and his Lord. So, whoever commits any of these abominations should conceal himself with Allah's concealment. Marriage to a woman from the People of the Book is permissible, provided she is chaste, but marriage to this particular girl is not permissible due to her involvement in adultery. It is advisable to seek a righteous Muslim woman. Furthermore, there should be no relationship of friendship between a man and a strange woman, as this constitutes the forbidden "mukhādanah" (illicit companionship).