If you have been smoking just a year or two, I would take richka's advice. If you have been smoking a long time--like 10 years--I actually think you should start with the easier stuff before you tackle the smoking. Smoking is the single hardest drug to quit. It's also the drug that is most worth quitting. That should definitely be your goal. But make a plan first and stick to it.
Eat better, get regular exercise. But don't beat yourself up if you fall off the wagon. It's tough when you don't feel good. Just try again the next day. One day at a time.
So, I would start by tapering off the chocolate, since that can contribute to headaches and is causing weight gain, which you need to slow down. When you are off the chocolate completely, tackle the headache pills--since you are probably getting rebound headaches. Taper off. Don't quit all at once.
After you are eating better, walking or exercising regularly and off the chocolate and headache pills, start tapering off the tobacco. This is going to be hard and I won't give any advice because I've never quit smoking, but I have a healthy respect for how hard it is. Just remember that millions of people have quit smoking and you can too. If you relapse, don't beat yourself up, just quit again. You WILL succeed!
When you feel you are really off the cigarettes, THEN worry about your weight and focus more on better diet, more exercise. Get away from things that make you sedentary, tv, computers, cars, etc.
Good luck!