Where do cells come from?

Cells come from pre-existing cells. This is one of the basic tenets of the cell theory. Earlier it was believed that the cells can arise spontaneously, however, this was proven wrong and the view has been rejected. Now we know that the cells can only arise from already existing cells, through the process of cell division. In this process, a cell divides to form two daughter cells and this way, new cells are generated.

One may question, where the very first cell (or cells) came from, if every cell comes from a pre-existing cell. The origin of first cells is difficult to trace, since there are no fossils of single cells. However, this is clear that various macromolecules organized themselves and grouped together to form the original cells. 

Hope this helps.