It may not sound true to most people, but it's my belief that most cases of obesity and weakness revolve around some kind of nutritional deficiency.
Quite frankly, I believe that there are some foods that people don't eat enough of, and are displaced by processed, junk foods that don't do much more for you other than stimulate your hunger and encourage you to eat more.
If you were to only make one small step toward a better overall dietary lifestyle to start making dramatic changes in the long term, it would be this:
Get 200 grams of animal-based protein a day.
That may be more than a lot of you need, but if you're a healthy person there is no unhealthy upper limit of protein, and the extra benefit is that you won't have room in your stomach for a bunch of other crap you don't need.