Be truly sorry for your sins.
The essential act of Penance, on the part of the penitent, is contrition, a clear and decisive rejection of the sin committed, together with a resolution not to commit it again, out of the love one has for God (which is reborn with repentance). Understood in this way, contrition is, therefore, the beginning and the heart of conversion, of that evangelical metanoia which brings the person back to God like the Prodigal Son returning to his father, and which has in the sacrament of Penance its visible sign, and which perfects attrition (imperfect contrition-born of the consideration of sin's ugliness or the fear of eternal damnation and other penalties threatening the sinner).2
The resolution to avoid committing these sins in the future (amendment) is a sure sign that your sorrow is genuine and authentic.
This does not mean that one has to promise never to fall again into sin. A resolution to try to avoid the near occasions of sin suffices for true repentance. God's grace in cooperation with the intention to rectify your life will give you the strength to resist and overcome temptation in the future.
Act of Contrition
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 they offend 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.
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.
You are now ready to go to Confession. The rite is presented on p. 336.
2. Cf. John Paul II, Apostolic Exhortation Reconciliatio et Pænitentia, 31, 1983.; cf. CCC, 1453.