If you're talking about death and heaven, then can I assume you are from a Judeo-Christian back ground. If not, I hope what I say is still somewhat helpful.
We have been talking to our oldest about this since he was very little. We always talked about how God loves us and wants us to have a very happy time while we are alive and that he wants us to go to heaven and be with him after we die. We talked about the fact that death is, in a way, a very happy thing because we know that person will be with God in heaven. (I don't personally believe in hell, so that doesn't concern me. My husband disagrees, but we try to focus on our belief that we try to live a good Christian life so this really shouldn't concern our son.)
We told our kids that one of the reasons people are so sad when someone dies is that they will really miss the person. They know that they will see them again in heaven after they die, but they will miss them until then.
He sometimes worries that his grandparents will die. We told him that they will and that we will see them again in heaven. But they are here now! We need to enjoy them as much as possible right now while they are still with us!
One of my favorite quotes from the Bible comes from the Gospel of John. "I came so that you might have life and have it more abundantly." John 10:10. God wants us to live our lives while we are here and enjoy eternity with him in heaven.
But if it helps, Albus Dumbledore said, "Death is but the next great adventure."