What causes skin cancer cells to go from normal to a cancer?

Ultraviolet light. While there is a new study implicating cigarette smoking as a contributing factor, most research points to ultraviolet light (sunshine, tanning bed, welding torch rays) as the most common trigger. More rarely, long term exposure to certain chemicals can also cause this in factory or farm workers whose skin is constantly exposed. A weakened immune system can aid the transformation.
Malignant mutation. . A genetic event that alters the normal cell cycle of a skin cell can transform it into a malignancy. Inherited faults of dna repair, along with tissue damage such as from inflammation or solar radiation can cause cells to make this transformation.