H.P.
I am no expert, but as a nurse and big health nut I can give some suggestions. As far as diet soda, the fake sugar actually makes you eat more food to compensate and Aspartame is not good in large amounts. Regular sodas, in small doses is actually better b/c since it has real sugar and cals, your body isn't making up for that loss in foods.
For meals, add a veggie or fruit to breakfast, lunch and dinner. Here's an example day:
Breakfast: 2 eggs, 2 pieces toast ( wheat or multigrain is best ), an orange, small yogurt
Lunch: turkey sandwich w/lettuce, tomato, cheese; apple; 1/2 cup carrots
Dinner: Baked chicken, 1 cup broccoli, brown rice
Dessert: 1/2 cup fro-yo w/ blueberries/strawberries
Also, drink lots of H20. You can replace needed caffeine w/ maybe a cup of tea or coffee, which is better than soda as long as you don't add too much sugar. And tea has good antioxidants that help fight off cancer.
Snacks: nuts (although high in fat, it's the good fats...just eat a handful), any fruit/veggie, multigrain chips if you really want chips, for sweet try raisins or nutella on toast
I don't think you need to do diet foods or be too strict. Just limit the very fatty/sugary things and minimize portion sizes.
Hope this helps! :)