What is the expiation for multiple sins committed, and what is the way to salvation from Hellfire and Satan?

The door of repentance is open to the adulterer and others, as long as the servant has not reached the state of ghirgharah (death rattle). Allah loves those who repent and forgives all sins for whoever repents from them. To remain steadfast in repentance: supplicate frequently and perform expiating good deeds, stay away from the causes and places of sin and bad company, seek good companionship that helps you obey Allah, and strive to get married while concealing past sins.