Before we were born, we believe that you, me, and everyone lived in Heaven--which we refer to as "pre-existence" or "pre-mortal Life"--as spirits. Our spiritual bodies looked similar to our physical bodies and we had the same gender. Jesus Christ was/is our eldest brother. 
 
We believe that in the Pre-existence, God, our Father, explained we would come to Earth to receive physical bodies and be tested so we can learn and grow and return to live with Him. Part of the plan was that Jesus Christ would come to Earth and fulfill the atonement. 
 
Lucifer, however, was prideful and did not agree with the plan. He offered his own plan, one that would take away our free agency. He tried to lead away spirits to follow him, and he became Satan. 
 
There was then a war in the Pre-existence. 1/3 of the spirits chose to side with Lucifer, the other 2/3 chose to follow God and His plan. 
 
Because of their rebellion, Lucifer and his follows were not allowed to be born on Earth and gain a body. Instead, they will forever remain spirits. 
 
Everyone who sided with God and Jesus Christ will at some point be born on Earth. That means you and me both chose to follow God and Jesus Christ in the Pre-existence. We were born and gained a body and are being tested so that we can learn and grow and become more Christ-like.