This is where so many Christians make themselves look so judgemental, and turn non-Christians off.
No, you do not go and tell someone to stop smoking! You discuss with them the pros and cons. You point out, in a non-personal way, the medical reasons for not smoking, and the different ways of combating stress and addiction.
And you can only do this if you have a fairly close relationship with them.
After all that, you have to leave it to them. Unless they are under-age children smoking, then they are adults and able to make their own choices. Laws have been passed in the UK to prevent people smoking in public places, which is as it should be - non-smokers do not wish to be forced to smoke second-hand smoke. More and more places are smoker-free, which again, is as it should be. There are offers of help all over the place for those who want to give up.
But if someone still wants to smoke, that is their choice, and they may do so at home.
It is the same with sin. You simply do not go up to a non-Christian and tell them they are "sinning". Their answer will, quite rightly, probably be "so what? What the **** has it got to do with you??"
There are layers and layers of sin in all of us. You might think, that when you first become a Christian, that all your sins are forgiven. Well, they are, but as you mature, you will see how sinful you truly are, and that "sin" is not a matter of things you do, or don't do, but your attitude towards God and life and other people.