When the priest says in confession, "Say your act of contrition," here is the standard response.
After confessing your sins to a Catholic priest, he will inevitably say something like, “Now you can say your act of contrition.”
This is a necessary part of confession, as it expresses your own “contrition,” or “sorry” for your sins.
The Catechism of the Catholic Church offers a brief explanation of this aspect of confession: “Among the penitent’s acts, contrition occupies first place. Contrition is ‘sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again.'”
Strictly speaking there is no “right” way or “correct” act of contrition.
However, the Church does give a few options. Here is the standard one in the Compendium of the Catechism of the Catholic Church.
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.
Or you can simply recite a verse from the Bible.
Father, I have sinned against you and am not worthy to be called your son. Be merciful to me, a sinner. Luke 15:18, 18:13
Whatever prayer you use, be sure that it comes from the heart.