I had read biology in childhood only, but as far as I can remember every living thing/being is made up of cells in the lowest level. Cells again are made up of some particular composition of molecules. But my question is how a particular composition of molecules become alive? Or What's that which turn compositions of molecules sentient? I'm trying to understand when or how insentient compositions of molecules become sentient.
Science does not have an answer to how the first reproducing cell came about. The actual event has left no traces except life as we see it today. There are theories about parts of how it might have happened, but evidence about it essentially doesn't exist. If you want to get a flavor of recent ideas of scientists in this area, here is a video of a lecture on "The Origin of Cellular Life on Earth".