What are the main causes cancer? how can you prevent it?

Many: The main cause is dumb luck, random genetic mutations. There's nothing you can do about your heredity, and different tumors run in different families. The single best cancer prevention is avoiding tobacco. Some HPV strains, and abusing alcohol are risks. Most "pop" advice about diet, chemicals and so forth is misguided. Staying fit is probably of some help. Best wishes.