One of the tenets of the LDS church rhymes with one of the laws of physics – nothing is created from nothing. Granted how that affects us religiously isn’t widely known, but it was one of the teachings of Joseph Smith, the founder of the LDS Church.
Just as God organized the world from unorganized matter, He organized our spirits as his children from “intelligences”. We could not have had this progression on our own, hence all of us being the spirit children of God the Father (including Jesus and yes, Lucifer). We were given the choice on how we would progress next – attaining a physical body and learning and growing on this Earth – through one of two options. The plan put forward by Jesus gave us the choice of how we would live our lives, even though we might not return to the Father, and the plan put forward by Lucifer was that we would have no choice, but all would return to the Father. The conflict between these two plans resulted in those following Jesus continuing to grow and those following Lucifer stopped from their growth (aka damned).
So, we are now in this mortal life, hopefully learning and growing and striving to become more like our Father. As we decide to (or not to) progress here determines if we will be able to progress after this life, to become like God and start on the same cycle, becoming Fathers and Mothers to our own spirit children but remaining children to our Father.
What we were before we were intelligences and what God the Father will be after we have become Gods we don’t know. I think one of the reasons its not discussed often is that it really doesn’t matter to what we should be doing now. Our mortal experience relies in the simple questions – where did we come from, why are we here, where do we go from here.
In short, just as God has always been and always will be, we also have an eternal existence.