Not only does it [the Sacrament of Reconciliation] free us from our sins but it also challenges us to have the same kind of compassion and forgiveness for those who sin against us.  We are liberated to be forgivers.  We obtain new insight into the words of the Prayer of St. Francis: "It is in pardoning that we are pardoned."

Jesus entrusted the ministry of reconciliation to the Church.  The Sacrament of Penance is God's gift to us so that any sin committed after Baptism can be forgiven.  In confession, we have the opportunity to repent and recover the grace of friendship with God. It is a holy moment in which we place ourselves in his presence and honestly acknowledge our sins, especially mortal sins.  With absolution, we are reconciled to God and the Church. The Sacrament helps us stay close to the truth that we cannot live without God. "In him we live and move and have our being" (Acts 17:28).

-USCCB

 Before celebrating the Sacrament of Penance, one should prepare oneself with an examination of conscience, which involves reflecting prayerfully on one's thoughts, words, and deeds in order to identify any sins.

There are various types of examinations of conscience but regardless of which one you use to prepare yourself for the Sacrament it should be rooted in Scripture; particularly, the Ten Commandments and Beatitudes. Below is an example of an Examination of Conscience that can help you prepare for the Sacrament.

Adult Examination of Conscience 

Children Examination of Conscience
 

O my God, I am heartily sorry for having offended Thee, and I detest all my sins because of Thy just punishments, but most of all because they offend Thee, my God, Who art all-good and deserving of all my love. I firmly resolve, with the help of Thy grace, to sin no more and to avoid the near occasions of sin.

or

My God, I am sorry for my sins with all my heart. In choosing to do wrong and failing to do good, I have sinned against you whom I should love above all things. I firmly intend, with your help, to do penance, to sin no more, and to avoid whatever leads me to sin. Our Savior Jesus Christ suffered and died for us. In his name, my God, have mercy.