I was trying to find the importance of the priesthood. Many other churches say everyone has it. How do you get it? Why is it so important?
The answer to your question can be best expressed by the Fifth Article of Faith:
We believe that a man must be called of God, by prophecy, and by the laying on of hands by those who are in authority, to preach the gospel and administer in the ordinances thereof.
In order to a) preach the gospel, or b) administer in the ordinances thereof, a man must be “called of God”. In other words, he needs a specific kind of divinely granted power and authority, or what we call “priesthood”. Priesthood is obtained after one has been, first, called of God by prophecy (meaning designated through the Spirit of Prophecy as a person who should have this authority); and second, has experienced “laying on of hands” (in other words, has gone through an ordaining ritual, performed by another righteous person who already wields this same authority).
N. Eldon Tanner had this to say about this importance of the Priesthood:
I would like to tell you how important this priesthood is. The Lord thought it was important enough to have it restored here upon the earth and have the Church reestablished. He thought it so important that he sent John the Baptist to come and restore the Aaronic Priesthood. Now John said as he met these two young men that he had been sent by Peter, James, and John—he was sent under their direction. He said, “Upon you my fellow servants, in the name of Messiah I confer the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins; and this shall never be taken again from the earth, until the sons of Levi do offer again an offering unto the Lord in righteousness.” (D&C 13.) How important it is! The priesthood of Aaron holds the keys of the ministering of angels and of the gospel of repentance and of baptism by immersion for the remission of sins. To think that a young man can perform those things, those ordinances as a member of the Aaronic Priesthood. Later, Peter, James, and John came and conferred the Melchizedek Priesthood upon them. Now that’s how important the priesthood is.
Many people can, and do, attempt to teach the Gospel and even perform ordinances of the Gospel (such as baptism) without having proper priesthood authority. Their efforts may make a positive spiritual impact in the lives of others. However, true priesthood offers a guarantee that religious ordinances are truly being performed under divine authority; and a man who receives the priesthood will find that his power to preach the Gospel effectively and to work miracles in the name of Jesus is increased.
Many of the saving ordinances we require for exaltation must be done by one who has the proper authority (Priesthood keys) to perform them. That’s how important the Priesthood is.
Now as to how you get it, worthy male members may receive the Aaronic Priesthood at the age of 12. These young men are ordained to the Aaronic Priesthood by one who has the authority (higher Priesthood) to perform such ordinations.