Question

how do some cells become brain cells and others become skin cells

Answer 1

cell differentiation

It happens when the zygote is developing, and different cells differentiate within one of the 1-3 layers (ectoderm, mesoderm, endoderm). Having all three means the organism is triploblastic, but some have only two of the three.

Answer 2

As for how, signals are formed. Though, like going into the specific layers, I don't know if you want or need that detail. Google cell differentiation for specifics.

Answer 3

well thank you very much that helped alot