The priest will generally give some advice to the penitent and impose a penance. Then he will ask the penitent to make an act of contrition. The penitent may do so in his or her own words, or may say any one of the acts of contrition from the Rite of Penance such as:
Act of Contrition
"O 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." (No. 45)
or
"Lord Jesus, you chose to be called the friend of sinners. By your saving death and resurrection free me from my sins. May your peace take root in my heart and bring forth a harvest of love, holiness and truth." (No. 90)
or
"Lord God, in your goodness have mercy on me; do not look on my sins, but take away all my guilt. Create in me a clean heart and renew within me an upright spirit." (No. 92)
or
"Lord Jesus, Son of God, have mercy on me, a sinner." (No. 92)
Source: http://forgiven.dor.org/about-this-site/act-of-contrition
