I want to know about my death. What is death? I want to experience my death. I want to be aware of my death.

As long as the life-force (‘Prana’) is inside your body, you are said to be alive. The moment that the Prana leaves your body to take another body is called death.
The death of the body is not the real death. Even in medical science, it has been proved that real death is not the death of the body, but the death of the mind.
From a spirituality perspective, real death is when you realize God, or realize the Self within you. That is because, it is only after then that you are not born anymore. Hence that is the real death.

Does a child miscarried at three and a half months go to the after life in the world of spirit?

Yes, it goes to the appropriate abode.

Does the remembrance of the Lord at the time of death depend on our past karma or because of spiritual endeavors during this life?

The remembrance of the Lord at the time of the death is neither due to past karma nor due to the spiritual endeavors during this life. It is only due to the grace of the Guru or God.

Where will a human being go after his death?

After the body dies, the soul goes to different lokas (heaven or hell) based on the punya or papa and to the lokas of their ishta devata.