When is a Catholic obliged to confess his serious sins? How often should one go to confession?
Upon reaching the age of reason, a Catholic is obliged to confess his serious sins. The Church urgently advises the faithful to do this at least once a year. At any rate one must go to confession before receiving Holy Communion if one has committed a serious sin.
By "the age of reason", the Church means the age at which one has arrived at the use of reason and has learned to distinguish between good and bad.
Can I make a confession even if I have not committed any serious sins?
Confession is a great gift of healing that brings about closer union with the Lord, even if, strictly speaking, you do not have to go to confession.
In Taizé, at Catholic conferences, at World Youth Day celebrations - everywhere, you see young people being reconciled with God. Christians who take seriously their decision to follow Jesus seek the joy that comes from a radical new beginning with God. Even the saints went to confession regularly, if possible. They needed it in order to grow in humility and charity, so as to allow themselves to be touched by God's healing light even in the inmost recesses of their souls. (YOUCAT questions 234-235)
Dig Deeper: CCC section (1457-1458) and other references here.
Part 2: The Celebration of the Christian Mystery (1066 - 1690)
Section 2: The Seven Sacraments of the Church (1210 - 1690)
Chapter 2: The Sacraments of Healing (1420 - 1532)
Article 4: The Sacrament of Penance and Reconciliation (1422 - 1498)
VII. THE ACTS OF THE PENITENT ⇡
The confession of sins ⇡
According to the Church's command, "after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year."56 Anyone who is aware of having committed a mortal sin must not receive Holy Communion, even if he experiences deep contrition, without having first received sacramental absolution, unless he has a grave reason for receiving Communion and there is no possibility of going to confession.57 Children must go to the sacrament of Penance before receiving Holy Communion for the first time.58
56.
Cf. CIC, Can. 989; Council of Trent (1551): DS 1683; DS 1708.
57.
Cf. Council of Trent (1551): DS 1647; 1661; CIC, can. 916; CCEO, can. 711.
58.
Cf. CIC, can. 914.
Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church.59 Indeed the regular confession of our venial sins helps us form our conscience, fight against evil tendencies, let ourselves be healed by Christ and progress in the life of the Spirit. By receiving more frequently through this sacrament the gift of the Father's mercy, we are spurred to be merciful as he is merciful:60 Whoever confesses his sins ... is already working with God. God indicts your sins; if you also indict them, you are joined with God. Man and sinner are, so to speak, two realities: when you hear "man" — this is what God has made; when you hear "sinner" — this is what man himself has made. Destroy what you have made, so that God may save what he has made. ... When you begin to abhor what you have made, it is then that your good works are beginning, since you are accusing yourself of your evil works. The beginning of good works is the confession of evil works. You do the truth and come to the light.61
59.
Cf. Council of Trent: DS 1680; CIC, can. 988 § 2.
60.
Cf. Lk 6:36.
61.
St. Augustine, In Jo. ev. 12,13:PL 35,1491.