Listen to the words of the priest. He will assign you some penance. Doing the penance will diminish the temporal punishment due to sins already forgiven. When invited, express some prayer of sorrow or Act of Contrition such as: O my God, I am heartily sorry for having offended you and I detest all my sins, because I dread the loss of heaven and the pains of hell. But most of all because I have offended you, my God, who are all good and deserving of all my love. I firmly resolve with the help of your grace, to confess my sins, to do penance and to amend my life. Amen.