I think sugar is the main problem (not fat) and I'm not convinced calory count is key. We did not evolved to eat processed sugar, which is not easily found naturally in the environment.
My 2 cents: - eat as much vegetables as you want (learn to cook them, with a little bit of olive oil) - eat as much fish as you want (no need to cook! Low temperature baking, 1h at 70-80°, the best cooking you'll ever have) - eat meat in reasonable, "as-if-you-had-to-hunt-it-with-a-bow" quantities - ban every processed food, sauce, appetizer.... If you would not eat a spoon of every single ingredient of some food, don't eat it. - ban all added sugar, except (real) honey in reasonable proportions.
This implies to know/learn how to cook (not so hard but this is easier when the local/family culture allowed you to learn passively).
It looks like this is hard to do in the USA: you don't easily find, for example, yogurt without sugar added. (Or I didn't look at the right place, once again this is not judgemental).
Generally speaking, it is easy to find online high-level cooking courses, but hard to learn the basics of how to cook your onions or tomatoes in different ways in everyday life, or make an healthy meal with what's left in the fridge; this could be interesting to have.
--edited for typing errors