"Let Us make man" is emphatic. It emphasizes the majesty of the speaker (the Lord of the universe!). Furthermore, the use of a plural for God allows for the later revelation of the Trinity.
Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit
Keep in mind, the "Us" cannot refer to the angels that are present with the Lord God because man is made in the image of God alone.
Genesis 1:26-27 Matthew 28:19