The Sacrament of Penance, commonly called Confession, is one of the seven sacraments recognized by the Catholic Church.  As Catholics, we  believe that all the sacraments were instituted by Jesus Christ himself.  In the case of Confession, that institution occurred on Easter Sunday, when Christ first appeared to the apostles after his Resurrection. Breathing on them, he said:  Receive the Holy Spirit. For those whose sins you forgive, they are forgiven; for those whose sins you retain, they are retained. (John 20:22-23)
 
The Marks of the Sacrament
As Catholics, we also believe that the sacraments are an outward sign of an inward  grace. In this case, the outward sign is the absolution, or forgiveness of sins, that the priest grants to the penitent (the person confessing his sins); the inward grace is the reconciliation of the penitent to God (which is why the sacrament is also sometimes called the  Sacrement of Reconciliation.
 
The Purpose of Confession
That reconciling of man to God is the purpose of Confession. When we sin, we deprive ourselves of God’s grace. And by doing so, we make it even easier to sin some more. The only way out of this downward cycle is to acknowledge our sins, to repent of them, and to ask God’s forgiveness. Then, in the Sacrament of Confession, grace can be restored to our souls, and we can once again resist sin.
 
Why Is Confession Necessary?
Non-Catholics, and even many Catholics, often ask whether they can confess their sins directly to God, and whether God can forgive them without going through a priest. On the most basic level, of course, the answer is yes, and Catholics should make frequent  acts of contrition, which are prayers in which we tell God that we are sorry for our sins and ask for His forgiveness.
 
But the question misses the point of the Sacrament of Confession. The sacrament, by its very nature, confers graces that help us to live a Christian life, which is why the Church requires us to receive it at least once per year. (Refer to The Precepts of the Church for more details.) Moreover, it was instituted by Christ as the proper form for the forgiveness of our sins. Therefore, we should not only be willing to receive the sacrament, but should embrace it as a gift from a loving God.
 
What Is Required?
Three things are required of a penitent to receive the sacrament worthily:

1. The penitent must be contrite, or, in other words, be sorry for his sins.
2. The penitent must confess those sins fully, in kind and in number.
3. The penitent must be willing to do penance and make amends for his sins..

How Often Should You Go to Confession?
While Catholics are only required to go to Confession when they are aware that they have committed a mortal sin, the Church urges the faithful to take advantage of the sacrament often. A good rule of thumb is to go once per month. (The Church strongly recommends that, in preparation for fulfilling our Easter Duty to receive Communion, we go to Confession even if we are aware of venial only.)  The Church especially urges the faithful to receive the Sacrament of Confession frequently during  Lent, to help them in their spiritual preparation for Easter.