Why does God use the pronoun us in Genesis 1:26?

In Genesis 1:26 God said let us make man in our image after our likeness. Who is the us and the our image and the our likeness referring to?

Genesis 1 : 26 And God 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 fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.