In every time that the true Church of Jesus Christ has been on the earth, God has chosen a prophet to lead it. In Amos 3:7, we read, "Surely the Lord God will do nothing, but he revealeth his secret unto his servants, the prophets."
There have been many prophets, Adam, being the first, Enoch, Noah, Abraham, Moses, etc. When Christ was on the earth, he set up the church with twelve apostles. After he was crucified, the apostles were all killed, people changed His teachings, and because of wickedness, the fullness of the gospel with priesthood power was taken from the earth. When this happens, it causes a period of spiritual darkness, called Apostasy. People are no longer able to be baptized by the proper priesthood authority.
When it is time to bring the fullness of the gospel back to the earth, the Lord calls a prophet to lead the church. Prophets are given the Priesthood authority and tell us the things that Jesus wants us to know.
Joseph Smith was one of those prophets. He restored Christ's true church here on the earth in the latter days. He was given the power of the Aaronic Priesthood by John the Baptist, who also baptized Jesus. He was also given the power of the Melchizedek Priesthood by three of Jesus Christ's apostles, Peter, James, and John. Because of these Heavenly Messengers, the proper Priesthood authority for baptisms, receiving the Gift of the Holy Ghost, and being sealed in temples with our families forever, is now on the earth.
There are many bits and pieces of the true gospel today, but I am so grateful that the Fullness of the Gospel of Jesus Christ has been restored to the earth, and that the Lord has given us a prophet to lead and guide us today: Thomas S. Monson. This has brought great light, knowledge, peace and joy into my life.