Best way to start getting healthy?

I am in a terrible way. I'm 21 and cant even run up a flight of stairs without gasping for breath or anything. Although i have a healthy BMI of 22.5, my choices of lifestyle are leaving me feeling worn out, sluggish and generally unmotivated to do anything. I smoke 20 cigs a day, which will be the first thing to go, and i know how to do that. its the whole when to start exercising, best exercises and how much, what food to eat??? i would be grateful for any advice on starting out in fitness and nutrition. cheers guys x