The Catechism of the Catholic Church on Penance & Reconciliation

Not only does it [the Sacrament of Penance] free us from our sins but it also challenges us to have the same kind of compassion and forgiveness for those who sin against us.  We are liberated to be forgivers.  We obtain new insight into the words of the Prayer of St. Francis: "It is in pardoning that we are pardoned." Jesus entrusted the ministry of reconciliation to the Church.  The Sacrament of Penance is God's gift to us so that any sin committed after Baptism can be forgiven.  In confession we have the opportunity to repent and recover the grace of friendship with God. It is a holy moment in which we place ourselves in his presence and honestly acknowledge our sins, especially mortal sins.  With absolution, we are reconciled to God and the Church. The Sacrament helps us stay close to the truth that we cannot live without God. "In him we live and move and have our being" (Acts 17:28).  

~from the United States Catholic Catechism for Adults

Been a while since your last confession? Use these Confession Guides:
Confession Guide for Adults  --  Confession Guide for Children
Examination of Conscience for Teens
Why should I confess my sins to a priest?
Act of Contrition
O my God, I am heartily sorry for having offended you,
and I detest all my sins, because of Your just punishments,
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 sin no more and to avoid the near occasion of sin.