You know the cell that multiplied to form you, so how does it split itself apart to form so many varieties of cells, won't it be the same cell every time it multiplies?

In early embryonic development—a morula like this—the cells are more or less genetically identical.

As cells proliferate, segregate into different functional groups, and form the first tissue layers (ectoderm, mesoderm, endoderm), and then those tissue layers differentiate into separate organs, some genes are silenced and others are activated (expressed). Even though different cells have the same genes, each cell type uses only some of them and silences others, leading to the variety of cells, tissues, and organs in the developed body.