Well first decide on your thesis. That means a statement giving your viewpoint. Then let the main body of your essay prove the dangers of smoking to the person smoking, and then the dangers to the family, and then how it affects everyone around you. You can list some statistics of lung cancer, tongue cancer, mouth cancer, black lung disease and all the illnesses that cigarettes and nicotine cause; the poor children who live with second-hand smoke, etc.
To end up, summarize your opinion and end like you started. A health tax could be charged to the cigarette companies. The people would pay more for cigarettes, and the government would take the tax from the industry.