I think this is an easy one. Start with the exercise. Let me explain my reasoning. If you combine your exercise with some simple affirmations and listening to motivational messages, you start to change your thought patterns. The combination of improving your health and the increase in motivation helps you transition to eating healthier with almost zero effort. The opposite is not as easy to do. Eating healthy does not give you the same ability to feed your mind at the same time, but changing your eating pattern first does have one advantage. When you eat properly, you start generating more energy making exercise easier. The only problem is with the improper mindset, you may not choose to use that energy for exercise, but may just waste it doing almost nothing. Search for a few highly motivating songs, a few motivational speeches, and write down a few affirmations that will empower you. You can then start exercising on a daily basis while saying your affirmations or listening to your motivational materials. Remember to stay focused on your goal of transitioning to eating healthier, also. You will find yourself making better food choices with almost zero effort and probably without even thinking about it.