All organisms begin as a single cell, even if as adults they will have billions of cells. That first, single cell splits. Then those two cells split, and so on.

When cells split, something special happens. First the cell makes an extra copy of all its genetic information. Then the information, stored in chromosomes, moves to opposite ends of the cell. Finally, the cell pinches itself in two. As cells split, a multicellular organism such as you grows!