The sacrement of forgiveness ( or of penitence or of reconciliation) is a volontary act of confession in front of a Priest in order to receive absolution i e the forgiveness of our sins.
This absolution is given by the priest when we sincerely regret our sins, and we express our firm desire not to sin again.
The first thing to do, before confession, is to consider our lives and to think about the times when our actions, our thoughts or our words have not corresponded to what God and the Church expect from us. We can equally sin by hurting those who live near us.
We are not talking about "whipping ourselves" or to state how overall bad we are, but just to allow a critical and realistic eye on our behaviour and our way of living. To confess is to admit that we are far from God, regrette this and to make some realistic resolutions not to do the same mistakes again.
In order to prepare oneself, it is good to begin by reading a passage from the Bible.