It is important to prepare a good confession - to be aware of one's sins and faults, and to have a heart ready to ask for God's forgiveness with the intention of true repentence.
Prior to going to confession, set aside prayerful time and use the Examination of Conscience in full and as a starting point to help you identify where you've fallen and need to be reconciled to God.
I am the Lord your God. You shall not have strange gods before me.
You shall not take the name of the Lord your God in vain.
Remember to keep holy the Lord’s Day.
Honor your father and your mother.
You shall not kill.
You shall not commit adultery.
You shall not steal.
You shall not bear false witness against your neighbor.
You shall not desire your neighbor’s wife.
You shall not desire your neighbor’s goods.
Once it is your turn to confess, begin by stating: "Forgive me, Father, for I have sinned. It has been (state the amount of time) since my last confession and these are my sins..."
Proceed to state the sins that you called to mind during your examination of conscience, genuinely being sorry, and receive any feedback the priest has for you throughout.
After any dialgoue, the priest will prescribe you a penance that you should complete in a timely manner once you've exited the confession and if your sins have been forgiven, the priest will absolve you (free you from your sins by God's grace).
Go in peace, cleansed and renewed in Christ!