God is our Father in Heaven, the Father of our spirits. In the Godhead with Him are Jesus Christ and the Holy Ghost. They are distinct beings with distinct roles but are one in purpose. This truth, along with many others, was lost after the deaths of Jesus Christ and His Apostles, during a period known as the Apostasy.
The Lord began to restore these lost truths in the spring of 1820, when 14-year-old Joseph Smith prayed in a grove of trees near his home in Manchester Township, New York, to know which church to join. In answer to his prayer, he saw God the Father and Jesus Christ. The Prophet Joseph Smith later wrote of this vision: "One of them spake unto me, calling me by name and said, pointing to the other — This is My Beloved Son. Hear Him!" (Joseph Smith–History 1:17)
On February 16, 1832, the Prophet and Sidney Rigdon received a revelation. To introduce the revelation, they testified of Jesus Christ and God the Father: "And now, after the many testimonies which have been given of [Jesus Christ], this is the testimony, last of all, which we give of him: That he lives! For we saw him, even on the right hand of God; and we heard the voice bearing record that he is the Only Begotten of the Father." (Doctrine & Covenants 76: 22-23)