How DNA programs the first cell in womb into a human
The short answer is: "Nobody knows yet." The slightly longer answer is that different species use different mechanisms. In amphibians and insect the oocyte, or egg, produced by the mother, has a polarity such that some macromolecules are enriched in different regions of the egg. The best characterized examples are localized RNAs, but there could be other signals as well.  In nematodes the site of sperm entry at fertilization determines the polarity of the cells, which then leads to asymmetry, etc. 

In mice, our closest approximation to humans, the early cells after fertilization all seem equivalent, but as the number of cells increases the cells on the inside of the ball and the cells on the outside start to choose different differentiation pathways. There can also be signals from the extraembryonic environment (i.e., the womb).