Confirmation is the Sacrament in which the Holy Spirit comes to us in a special way to join us more closely to Jesus and his Church and to seal and strengthen us as Christ’s witnesses. It is the completion of Baptismal grace.
— Catechism 1285

The Catechism of the Catholic Church also discusses Confirmation in verse 1316.

When the intercessor comes, whom I will send to you from the Father — the Spirit of truth that comes forth from the Father — he will bear witness to me, but you too will bear witness.
— Gospel of John 15:26

For more description of confirmation, see the Acts of the Apostles 2:4 and the Acts of the Apostles 8:14-17.

To confirm means to strengthen. In Confirmation our faith is deepened and strengthened, and through this Sacrament we are more perfectly bound to Christ and to His Church
— Catechism 1285

Who can receive Confirmation?

Any Baptised Catholic who has not been Confirmed may receive the Sacrament. The Church urges Catholics to study their faith well before being Confirmed because a confirmed Catholic is expected to live up to his or her faith
— Catechism 1306

Is there a preferred age for Confirmation?

In the Latin tradition the person to be Confirmed should have reached the age of discretion. However, since the Sacrament is a free gift, one need not be any specific age in order to receive it. In the Eastern Church, Confirmation is given to infants immediately after Baptism.
— Catechism 1307-1308

How should a Catholic prepare for Confirmation?

A Catholic should prepare for Confirmation by studying the Catholic faith, by praying and by witnessing to Christ. The candidate must be in a state of grace and have the same intentions as the Church.
— Catechism 1309-1310
They devoted themselves single mindedly to prayer.
— Acts of the Apostles 1:14

What is the purpose of a Confirmation Sponsor?

A Confirmation sponsor has the same duties as a Baptismal godparent. The Church suggests that the Baptismal godparent become the Confirmation Sponsor, however, another sponsor may be chosen. A sponsor must be a devout, practising Catholic who is already Confirmed.
— Catechism 1311