The instant before He creates man, God reveals what it is like to be the eternal God. The Genesis creation account lets us “hear” God speaking to Himself: “Let Us make man in Our image” (Gen 1:26).
This was His customary way of presenting His own reflecting on His creation, using singular and plural at the same time: “Then the Lord God said, ‘Behold, the man has become like one of Us’” (Gen 3:22).
When God calls Isaiah to be His prophet, He expresses this same relationship within Himself again: “Then I heard the voice of the Lord saying, ‘Whom shall I send. And who will go for Us?’” (Isaiah 6:8)
In fact, God began to present Himself this way even earlier, in the very first words of His revelation: “In the beginning God created the heavens and the earth” (Gen 1:1).
Every Hebrew-speaking person knew that their word for “God” Elohim was a plural form.
But notice what The Lord declares about Himself when He introduces His Law to the nation of Israel: “Hear, O Israel! Yahweh (I AM) is our God (Elohim), Yahweh (I AM) is One!” (Deut 6:4)
So, from the beginning, God wants us to understand that He is One and, at the same time, plural.
(Spotlight 1, Lesson 3 in Doctrine 101: Learning about God)