Be realistic and set small goals for yourself that are achievable. Transition your diet as you run out of a conventional item, replace it with a healthy version. Begin by avoiding the 6 evils of processed food:
No white sugar
No white flour
No high fructose corn syrup
No hydrogenated oil
No artificial colors or flavors
Try to avoid juices, chips, cookies and the vice of your choice (mine is chocolate). These "snacks" are empty calories and add a lot of sugar to your diet.
lean sources of protein (beans, tofu) (If you eat meat: turkey, fish, chicken)
Try to eat 6 small meals a day that are balanced (salad, olive oil dressing, lean protein, and a fruit).
Allow yourself to eat some of your favorite foods, just cut the portion back and really enjoy it (be present).