The first Sacrament is the Sacrament of Baptism. It is through Baptism that all sins are forgiven and we become co-heirs with Christ.
Baptism is necessary for salvation, as Jesus says in John 3:5, unless we are born again of water and the Spirit we cannot know eternal life. But, it’s important to note, God bound salvation to the sacraments but God is not bound by them.
God’s plan always included the sacraments, and this is seen when we recognize the Old Testament prefigurations. For example, the flood of Noah and Moses parting the Red Sea are both prefigurations of Baptism and how God saves us through water.
In the Roman Catholic tradition, the sacrament is valid when it’s said in the Trinitarian formula (in the name of the Father, the Son, and the Holy Spirit), as Jesus instructed in Matthew 28:19-20.