There are seven sacraments in all but the first one that we receive is Baptism. Baptism is the asis of Christian life. We are all born sinners, none of us are saved by default. Christians must be born of the water and the spirit. This process brings about a rebirth into the Christian life. The other sacraments include Confirmation, The Eucharist, Confession, The Anointing of the Sick, Holy Orders, and Marriage. All seven come together to complete the sacraments.
However, Baptism can easily be seen as the most important because it i the first sacrament upon which one can build foundation for the others.