Smoking is one of the most dangerous things that a person can do to themselves. And unfortunately for a lot of the people who practice this deranged habit, it is often extremely difficult to see the problems which will develop as a consequence of it until it is too late to do very much about it. After all, lung cancer, emphysema and just the difficulty with breathing (which is a very necessary activity to continue doing if you like to live) are usually long term problems which can literally take years, or even decades to form inside of a smoker. If a young person were to start smoking today, they might be in their forties or even fifties before the problem becomes serious enough that they can not deny it any longer. And to a young person who already feels pretty invincible, it can be very hard to get them to understand the long term consequences of anything.
When you talk to your kids about smoking, it is going to be extremely helpful to you if you have already instilled in them an appreciation of the future, and how the actions that they do today are going to influence the way that their lives are going to go. If they do not care much about the future, anything that you tell them is going to happen at some point going forward is essentially going to fall on deaf ears. Why would a person who lives in the moment care about what is going to happen in a couple of decades?
However, you can tell them about how expensive cigarettes are, and how easily they can save up their money to buy neat things in the fairly near term future if they avoid it. You can also mention to them that smoking does nasty things, not only to their lungs (and their ability to play sports at their best potential), but to their teeth, fingernails and clothes. In a lot of ways, the near term stuff is not that great, either. Hopefully they will make the best decision for their lives.