The King Follett Discourse was a sermon given by Joseph Smith in April 1844 during a general conference of the Church. It was delivered after the death of a Church member named King Follett and was meant to offer comfort while also explaining important doctrines. In the sermon, Joseph Smith taught about the nature of God, the purpose of human life, and the idea that God was once like us. It remains one of his most well-known and detailed teachings about who God is and who we can become.
… I am going to tell you how God came to be God and what sort of a being He is. For we have imagined that God was God from the beginning of all eternity. I will refute that idea and take away the veil so you may see. Truth is the touchstone. These things are incomprehensible to some, but they are simple. The first principle of truth and of the Gospel is to know for a certainty the character of God, and that we may converse with Him the same as one man with another, and that He once was a man like one of us and that God Himself, the Father of us all, once dwelt on an earth the same as Jesus Christ himself did in the flesh and like us.
What kind of a being was God in the beginning, before the world was? I will go back to the beginning to show you. I will tell you, so open your ears and eyes, all ye ends of the earth, and hear, for I am going to prove it to you with the Bible. I am going to tell you the designs of God for the human race, the relation the human family sustains with God, and why He interferes with the affairs of man. First, God Himself who sits enthroned in yonder heavens is a Man like unto one of yourselves—that is the great secret! If the veil were rent today and the great God that holds this world in its sphere and the planets in their orbit and who upholds all things by His power—if you were to see Him today, you would see Him in all the person, image, fashion, and very form of a man, like yourselves. For Adam was a man formed in His likeness and created in the very fashion and image of God. Adam received instruction, walked, talked, and conversed with Him as one man talks and communicates with another.
Larson, S. (1978). The King Follett Discourse: A Newly Amalgamated Text. BYU Studies, 18(2), 193-208. Retrieved from http://www.ldslearning.org/lds-king-follett-discourse-a-newly-amalgamated-text-byu.pdf