Although Adam and Eve could no longer walk and talk with Heavenly Father as they had in the Garden of Eden, they could hear his voice. Heavenly Father spoke to them, giving them commandments. He told them to pray to him and make sacrifices to him—the firstlings of their flocks. So Adam killed an animal and burned it on an altar made of stone.
One day an angel appeared to Adam and asked him why he offered sacrifices. Adam told the angel that he did not know the reason for it, he only did it because Heavenly Father commanded it. Indeed, it is important that we follow Heavenly Father's commandments even when we don't understand the reasons for them, for whatever Heavenly Father commands is right, no matter what it is.
The angel explained the Plan of Salvation to Adam, and told him that the sacrifice of animals was to remind people of the future atoning sacrifice of Jesus Christ, Heavenly Father's firstborn child. Adam was taught that mortals must repent their sins and become baptized if they are to one day become gods. The angel baptized Adam, the Holy Ghost came upon him, and he received a vision of the future of all his offspring down to the last generation.
And so the Church was organized on Earth in the days of Adam, and he became its first president. Adam and Eve were the first Christians and they were the first to preach the gospel of Jesus Christ among their children. Adam ordained each of his obedient sons as priests, and his sons passed on the priesthood to their sons in the same way.