Normal cells transform into cancerous cell through accumulation of mutations. When the cell acquire mutations that enable their unlimited proliferation and random migration, it becomes a cancerous cell. Most of the time, those cells are detected and destroyed by our immune system. However, occasionally one or two might escape, proliferate and invade healthy cells surrounding them. They become cancer then.
Sharon’s answer is great for this question. Mutations cause normal cells to transform into cancerous cells. Also, according to the Cancer Genome Project, most cancer cells possess 60 or more mutations.