This is all about Penance and the Sacrament of Confession.
Interior penance is all about fasting, prayer and almsgiving . . . and not shaving . . . OK, you can shave if you have to.
Reading sacred scripture, praying the liturgy of the hours and the Our Father, and every sincere act of worship or devotion drive us to conversion and contribute to the forgiveness of our sins.
Sin is first and foremost an offense against God, but also damages the communion with the Church. We need forgiveness from both.
Going into confession we need to have a contrition: sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again (1451).
More on Catholic Confession here if you were curious: http://www.catholic.com/tracts/the-forgiveness-of-sins