It's easiest to look at this in three parts - where we were before this life, what we're doing in this life, and where we're going after we die.
Where did I come from?
Before we lived here on this earth, we lived with God in heaven. God is the father of our spirits. We are literally his children; that's why we call him "Heavenly Father." God loves us and wants us to enjoy all the blessings that he has; but we couldn't do that if we stayed in heaven and never got a body and never had to choose between right and wrong.
So God created a perfect plan that would enable us to come to earth, gain bodies, gain experience, and return to him. We understood and accepted the plan before we came here.
Under the direction of Heavenly Father, Jesus Christ created the earth as a place for us to live and gain experience. In order to progress and become like God, each of us had to obtain a body and be tested during a time of probation on the earth. While on the earth we are out of God’s physical presence. We do not remember our pre-earth life. We must walk by faith rather than by sight.
Why am I here?
We are here to get physical bodies, gain experience, and to use our agency to make the choices that will help us become like Heavenly Father. In a nutshell, we are here to become more like God.
Jesus Christ is central to this plan - Heavenly Father's plan. God knew that we wouldn't be perfect when we lived here, and we would make mistakes. But "no unclean thing" can dwell in his presence - so how could we return to him? This is where Christ comes in. It was always in God's plan that Jesus would come to the earth and live a perfect life so that he could redeem the rest of us - and he did. Because of Christ's perfect life and sacrifice, if we repent, his atonement pays for our sins and we are clean again. This is what enables us to return to live with Heavenly Father.
Additionally, Christ's resurrection (coming back to life) allowed each of us to also be resurrected - and to live again (in our perfected, physical bodies, never to die again) after we die.
Where am I going after I die?
For a time, before the judgement, we will live in what's called "the spirit world." This will be a place of peace and rest for those who chose to obey God during our mortal life. Those who did not obey God and did not repent will be unhappy. In the spirit world, the gospel is preached to those who did not obey the gospel and those who did not have the opportunity to hear it while on this earth. If they accept the gospel in the spirit world, it will be like as if they had accepted it here.
After a time, our bodies and spirits will be brought back together through the resurrection, and we will stand before God to be judged. We will remember everything we've done - both good and bad. If we've repented, we will receive mercy. Because God rewards us according to our works and desires, and everyone's not just all good or all bad, there are different kingdoms of glory to which me may be assigned after the judgement.
Those who continued in their sins and did not repent in this life will receive their reward in the lowest kingdom, which is called the telestial kingdom. This kingdom is compared to the glory of the stars.
People who do not accept the fulness of the gospel of Jesus Christ but live honorable lives will receive a place in the terrestrial kingdom. This kingdom is compared to the glory of the moon.
Those who have repented of their sins and received the ordinances of the gospel and kept the associated covenants will be cleansed by the Atonement of Christ. They will receive exaltation in the highest kingdom, also known as the celestial kingdom. They will live in God’s presence, become like Him, and receive a fulness of joy. They will live together for eternity with those of their family who qualify. In the scriptures this kingdom is compared to the glory or brightness of the sun.
And so that's God's plan for us - which, in the scriptures, is called a merciful plan, the plan of salvation, the plan of redemption, and the plan of happiness. God wants us to be happy, and wants us to be able to enjoy all of the blessings that he has.
"For behold, this is my work and my glory - to bring to pass the immortality and eternal life of man." - Moses 1:39
Resources you might be interested in:
- God's Plan of Salvation, a page explaining God's plan for us
- Mormon.org, a website filled with answers to frequently asked questions about The Church of Jesus Christ of Latter-day Saints
- Chat with the Missionaries, where you can live chat with Mormon missionaries and ask them any questions you want.