You should start with what has the greatest impact on you every day, and smoking is one of those factors. When I also decided to change my life, it helped me to realize how harmful habits affect the body. The article https://ways2well.com/blog/smoking-and-stem_cell-therapy-how-smoking-affects-healing-and-results clearly shows that smoking not only damages the lungs, but also significantly reduces the body's ability to recover from exercise, stress, and even injuries. This is a factor that undermines your chances of success in sports, nutrition, and overall well-being. It helped me: I quit smoking as a first step, and after a few weeks, I noticed that it was easier to run, I slept better, and I was less tired overall.