Heavenly Father convened a Grand Council in our premortal life and told us about the plan of salvation (plan of happiness) that required our coming to earth to receive a physical body and gain valuable experience. The plan was necessary so we could return and live with our Heavenly Father after our death on earth.
Since all of us would sin and make mistakes on earth, we needed a Savior to pay for our sins and be an example of how we should live our lives. The only question was who would be our Savior.
Jesus Proposed Himself as our Savior
Jesus expressed his willingness to come to earth and be our Savior. He wanted us to retain our agency, our freedom to make choices, and be able to choose the right path; because this would prove our worthiness for exaltation.
It was clear that some of us would not make right choices and would sin. However, our agency was so important this had to be risked.
He stipulated that although he was willing to do this, Heavenly Father would receive all the glory, or in other words, all the credit.
In addition, he was willing to accept whatever decision Heavenly Father made regarding who would be our Savior.
Lucifer Proposed Himself as our Savior
Lucifer also said he was willing to come to earth and be our Savior. He did not want us to retain our agency, he wanted to force us all to be good. By doing this, not one person would sin and all would redeemed and able to live with Heavenly Father again after death.
He stipulated that he was willing to do this. However, he wanted all the glory, or credit, for himself.
Jesus is Chosen and Lucifer Becomes Satan
After the two proposals, Heavenly Father chose Jesus to be our Savior. Not willing to accept this, Lucifer rebelled. He had the support of one third of all Heavenly Father's spirit children.
This rebellion is known as the War in Heaven. As punishment, Lucifer and his followers were cast out of heaven and were not eligible to come to this earth and have physical bodies
Now known as Satan, Lucifer and the spirits that followed him are also on this earth. They tempt and persuade us to do evil and make wrong choices.
Since we are here on earth and have bodies, we know we sided with Jesus and chose him to be our Savior. This is sometimes called receiving our first estate. To receive our second estate we need to live righteously in this earth life.
We Should Follow the Savior's Example and Teachings
When the first spirits were sent to earth, Jesus Christ provided them with his gospel. Eventually, he came to earth himself, where he paid for our sins and became our Savior.
Also while on the earth, he taught us and guided us on how we should live our lives here on earth. He set up his church and the authority to act in his name. This authority is called priesthood authority.
He taught everything we need to know in order to live our lives here and live with our Heavenly Father after we die.
Over time, when some of his teachings were lost, he restored them by revealing them again to living prophets.
How Should Knowing All This Change My Life?
Since we knew and supported the plan of salvation in our premortal life, we need to learn it again here and support it here.
We need to use our agency wisely and make good decisions here on earth, so they we can return to our Heavenly Father and live with Him again.
We need to repent of our sins so that Jesus Christ's atonement can pay the price for us.
We need to be constantly aware that Satan and his followers are trying to make us sin.
We need to live our lives the way Jesus Christ did, learn from him and make right choices.