Reconciliation

Before celebrating the Sacrament of Penance, one should prepare themselves with an examination of conscience. 

An examination of conscience is a “prayerful self-reflection on our words and deeds in the light of the Gospel to determine how we may have sinned against God”  (Catechism of the Catholic Church). 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. The USCCB provides several Examinations of Conscience that can help you prepare for the Sacrament.

Act of Contrition (the simple truth)

"Lord Jesus, Son of God, have mercy on me, a sinner."

Or...
"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 occasion of sin. Amen."

Other Resources on Penance & Reconciliation are available on website of the United States Conference of Catholic Bishops.