Nicotine in cigarettes is an addictive drug therefore it is very difficult to give up smoking once you have started.
Here are some other very important reasons why you shouldn't smoke:
It costs lots of money (if you smoke one pack a day it will cost you £1,300 a year - just think what you could buy with that!)
It damages your heart and can cause cancer. (Did you know that, according to ASH, in the last fifty years around six million people in Britain will have died from smoking related diseases).