Please explain if it is the tobacco or what they add to it that gives you the cancer and how long would it take you to get cancer from smoking?

It's the tobacco: The products of combustion include chemicals that wreck havoc on the genes. After 15 pack-years, your risk is much higher than other folks and doesn't return to baseline for a few decades. More important, you've become a slave to an addiction that now carries a serious stigma in the civilized world. I can't promise you cancer, but it will take your wind away both now and as you get older.