My dad smokes, and I think that's really affecting his health. How do I get him to stop?

It's really nice that you care so much about your dads health. All you can do is share your concern with him - let him know how you are feeling and the health concerns you have. After that you have no control over the situation. It is up to your dad now to create the change and stop smoking. He has to be ready to quit - he has to be the one to want to do that. Remember that we have no control over other people's choices. The fact you have told him about it is enough on your part. There is nothing more you can do. Hopefully if he does decide to quit you will be there to support him :)

It's really great that you're taking a step towards helping a family member get off a possibly harmful activity. Although it might not be easy to convince someone to not do something because it's harmful to them - they know this already and still chose to do it anyway - especially when it's a parent, I think it depends on the kind of relationship you have with your dad as well. If you're close, and talk about everything to each other, and are mostly open in your communication, it would be relatively easier than if you weren't as close. If this is the first time you're approaching him, I'd suggest asking questions: about why/when he started, and if he tried to stop ever (even if you know the answers). Then, posing your concerns and making it so that he knows how much you're worried about his health might help. Talking it out with him is honestly all I can suggest. If that hasn't worked, try making him see how it's affecting his surroundings: you, the rest of your family, his friends, or whatever. Sometimes people stop doing things when they become more aware of how it affects the people around them.