Baptism is the Christian sacrament of Initiation into the life of Christ, both in the sense of identification with the person of Christ (in the more ancient mode of baptism, immersion, going under the water and rising again from it symbolically links the baptized person with the burial and Resurrection of Jesus) and also in the sense of participation in Christ’s sacramental body, the Church. Receiving baptism is the precondition for receiving any of the other sacraments.
In the Catholic tradition, infants and small children are baptized on the condition that their parents and godparents commit to raise them into Christian faith; since Baptism makes one a Christian, it is a matter of importance that anyone baptized by consciously initiated into the teachings and practice of Christian life.
If you are an adult who would like to be baptized, please refer to the page on the Rite of Christian Initiation. If you are a Christian who has been baptized outside of the Catholic Church, and are seeking full communion with the Roman Catholic Church, the Catholic Church will probably recognize your baptism as valid; however, before you are free to participate in receiving Catholic sacraments, you will first have to be accepted into the Catholic Church by the Sacrament of Confirmation and/or a statement of faith. Again, please refer to the page on the RCIA.