My thoughts:
Learn to cook, if you don’t know how already. You don’t need to become a great chef, but learn to follow a recipe because 90+% of your meals should be made with whole foods, i.e. stuff that came out of the ground instead of a box. Even “meal starter” packages can contain a lot of stuff that can sabotage you.
On that note, read labels. Read EVERY label. It’s very enlightening.
When in doubt, eat a non-starchy vegetable first. Include vegetables in everything. If you’re anything like me, even making an effort barely meets that standard “5-9 servings per day,” so it’s really difficult to go overboard here. I try to work with the “rainbow” principle too – eat veggies that come in a wide variety of colors, in order to get a broad range of nutrients.
If you can, completely cut out fast food, and processed foods like chips, cheetos, oreos, soda, etc. for six months (or more). Once you get past the “withdrawal” cravings, you will very likely lose much of your taste for them.
Instead, if you have a local farmers market or several, take advantage. The vendors very often have really great suggestions for how to use / prepare their produce, and if you can get into the wide variety and real freshness offered by produce that is practically straight out of the ground, you will tend to miss the processed foods a lot less. Get adventurous here.
Drink lots of water. I aim for at least 2 litres a day. I do exercise, so my trainer recommended up to 5 litres a day (I usually manage 3-4).
Try to cultivate a mindset that treats are special rarities. For example, I don’t drink Diet Coke or Coke Zero every day. But I might have the real deal “Mexican” Coke (made with cane sugar instead of HFCS) once in a month or two. It becomes extra special because it’s real and because it’s rare. Are oreos “special” or would you rather go to that nice restaurant and have their house-made tiramisu? Savor the pleasure of the treat rather than mindlessly snacking.
Savor the pleasure of all of your food, for that matter. Mindfulness can go a long way.