When God created human beings he said
“Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.” (Genesis 1:26)
In saying “Let us”, he appears to be speaking to someone.
Later in Genesis, after Adam and Eve sinned, God says
“Behold, the man has become like one of us in knowing good and evil.” (Genesis 3:22)
Again, God appears to be speaking with his heavenly angels who understood both good and evil.
The third example of this “us” usage is discussed in Who are the “us” in “let us go down” of Gen.11:7?
One traditional view among both Jews and Christians is that God is talking to his angels. The idea that there were there is supported by a few other passages in the Bible. A later passage which records the “sons of God” shouting for joy at creation (Job 38:7). However that verse only reflects the idea of angels watching, with no implication that there is any involvement in creation.
Another view is that this plural is simply rhetorical. God speaking to himself and the cosmos. This is supported by the actual making of man being by God (Elohim – singular noun, not plural) with the verb “made” in the singular form.