Matthew 28:10 By "brethren", Jesus probably meant His disciples. This showed that He had forgiven them, even after they had disowned and deserted Him. Their relationship would now be even stronger than before.
Matthew 28:10 Jesus told the women to pass a message on to the disciples - that He would meet them in Galilee, as He had previously told them (Mark 14:28). But the disciples, afraid of the religious leaders, stayed hidden behind locked doors in Jerusalem (John 20:19). So Jesus met them first there (Luke 24:36) and then later in Galilee.
Matthew 28:11-15 Jesus' resurrection was already causing a great stir in Jerusalem. A group of women was moving quickly through the streets, looking for the disciples to tell them the amazing news that Jesus was alive. At the same time, a group of religious leaders was plotting how to cover up the Resurrection.
Matthew 28:18 God gave Jesus authority over heaven and earth. On the basis of that authority, Jesus told His disciples to make more disciples as they preached, baptized, and taught. With this same authority, Jesus still commands us to tell others that Good News and make them disciples for the kingdom.