A person notices a dark spot on their skin. This dark spot turns out to be skin cancer; what could have caused

the start of the skin cancer tumour and how was mitosis and cell divisdon of these cells affected?

The root cause was probably exposure to uv light. The phenomenon of tanning is triggered by DNA damage in cells caused by uv light. The early mutations inactivate genes that function to control the rate of cell division and mediate DNA repair. After that, its all downhill, with cells dividing at a greater than normal rate and less capable of fixing additional damage.