The priest will propose an act of penance. The penance might be prayer, a work of mercy, or an act of charity. He might also counsel you on how to better live a Christian life.
After the priest has conferred your penance, you will be asked to make an Act of Contrition, expressing sorrow for your sins and resolving to sin no more.
A suggested Act of Contrition is: 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. (Rite of Penance, no. 45)