Two excerpts from the Catechism:
“Sin is before all else an offense against God, a rupture of communion with him. At the same time it damages communion with the Church. For this reason conversion entails both God's forgiveness and reconciliation with the Church, which are expressed and accomplished liturgically by the sacrament of Penance and Reconciliation.” CCC 1440
“Individual, integral confession and absolution remain the only ordinary way for the faithful to reconcile themselves with God and the Church, unless physical or moral impossibility excuses from this kind of confession. There are profound reasons for this. Christ is at work in each of the sacraments. He personally addresses every sinner: ‘My son, your sins are forgiven.’ He is the physician tending each one of the sick who need him to cure them. He raises them up and reintegrates them into fraternal communion. Personal confession is thus the form most expressive of reconciliation with God and with the Church.” CCC 1484
Upon entering the confessional, tell the priest how long it has been since your last confession, then simply and honestly confess your sins. No need for long explanations, the priest may ask for context if it becomes helpful. Serious sins should be confessed by type and frequency. (I have stolen $300 from my employer on two occasions / I got drunk about twice a month)
After this, you will receive an act of penance to complete later, and be given absolution from your sins. Those who confess with a humble heart are sure to walk out refreshed in body, mind, and soul! Do not be afraid.
