Fitness and diet is such a shallow topic it is genuinely baffling how much ink has been spilled over it.
If you want to gain mass, eat more, and eat like a farmboy with lots of dairy, eggs, and pork. If you want to lose weight, eat less, and pick shit that is obviously not junk food. If you want to to get in shape, pick any sport or strength training regime and train it consistently. It's only a matter of consistency and self-control, the easiest shit in the world.