* It needs to be tasty and satisfying, or you won't stick with it. Don't start with some extreme diet: e.g. "only raw food", "only stuff grown within 20 yards of your house", "No X where x is some macronutrient".
* Set a goal of eating zero processed foods. If it comes in a box, don't eat it. If it comes in a can and it isn't just preserved, pure veggies (e.g., canned tomatoes) or fish (e.g., sardines), don't eat it. What can be done in a typical kitchen is about the max amount of processing you should regularly tolerate.
Very few will argue with the above points, unless they have some sort of agenda or ideology beyond just eating well. If that's all you do, you'll be eating more healthily than the vast majority of Americans. You'll probably feel better.
If you want to go further, you can:
* If you're going to eat animals, stick to animals that eat a diet for which they've evolved. i.e., prefer grass-fed beef to corn-fed.
* Prefer a variety of plants, and prefer plants that have been raised without chemical fertilizers or insecticides. We don't have enough data to know, but it seems likely that plants have evolved all sorts of chemicals to protect themselves from their environment that may also help protect us from their environment.
The theme of the above two points is- "let's not fuck with the diet that kept humans alive since the development of agriculture and cooking. Let's let other poor saps be the experimental subjects"
I also prefer to do things whose benefits are less obvious, such as eating very little meat. That's likely healthy, but I do it more to be a decent neighbor.
That's my reasoning, and how I eat. Changing to eat that way was surprisingly easy and enjoyable, and I no longer feel like I need to change my diet. I physically feel better: fewer headaches, better sleep, more energy, better "digestion". When I started eating that way, I dropped 10 pounds without trying (I wasn't overweight to begin with, and I'm not underweight now).
Here are the downsides - I expect them to apply to any reasonable diet:
* It's more expensive.
* It's more time-consuming. I cook more. I enjoy it, but it is less convenient.
* It's a challenge to not come across as a snob, especially when around less urban folks. I most frequently compromise on my diet when with family members.
* It's hard to eat that way outside of real cities. Produce at major supermarkets is frequently awful. Chain restaurants sell preprocessed garbage.