Baptism is the Sacrament through which God introduces us to the Christian life. St Paul says that in baptism we are buried with Christ and now share his risen life (cf Rom 6:4). Baptism is the gateway to all the other Sacraments because through it we become children of God and followers of Christ. It is like a seed of faith sown in us that has to grow strong and bear fruit in our lives. That seed is nourished by the witness of other Christians but most especially by the grace God gives us in the other Sacraments.
Following an ancient tradition going back to the first Christians, the Catholic Church baptises both infants and adults. Children are baptised because of the strength of their parents' and godparents' faith and Christian life. Adults are baptised because of their own faith and desire to follow Jesus.