A few posts back, I wrote a post about authority, especially as it pertains to God's priesthood. If you'll remember, I compared it to a flame from a lighter lighting a candle. That represents the priesthood being given from God to Man. Once the candle has the flame, it can be used to light other candles. Just the same, once man has the priesthood, he can use it to give it to other men. However, if all of the candles burn down or are blown out, there can be no more flame until the lighter is used again. In exactly the same way, once the authority is lost it cannot be restored or found on the Earth until God acts to restore it.
The authority given to the Twelve apostles by Jesus Christ had been lost. The Apostles were killed, many of them in ugly ways. Nowhere on the entire earth was there a man who possessed the authority.
God had to restore the priesthood.
Who alive currently had the priesthood? Nobody. So, he had to send somebody who had been alive, but was not on the earth. Peter, James, and John, three of the apostles of Jesus Christ, came and bestowed the priesthood on Joseph Smith and one of his friends.
The priesthood was restored, and with it the church. Later, Moses, Elijah, and other angels came and gave Joseph the keys to the priesthood, and all was complete.
In our times, the church of God has been re-established. Now, as in days of old, we have twelve Apostles. Now, we have a prophet who speaks with God and receives revelations from him.
The only way to find out for yourself is to pray and ask God. He knows if it's true, doesn't he?