How does smoking affect the body?

When you inhale smoke, you get a smoke residue that coats your lungs and makes it harder to breathe; eventually this can cause emphysema and/or lung cancer. In general, smoking is very bad for your health. Don't do it.
The chemicals from the cigarettes get into your lungs and cause tar to build up. You get addicted to nicotine and your teeth turn yellow.

The long-term damage to lung tissue (including emphysema) and the risk of various forms of cancer, should be sufficient reasons for a decision not to smoke.