There is no one simple rule or method that leads to a good diet. Even if there were a formula, you'd probably get bored with it over a lifetime of eating, so it needs to kind of a living work that's open to evolving and being reinvented. (And only you know exactly what you like.)
There's a lot of good basic advice out there about eating healthy foods (more vegetables, more fiber, less sugar, don't binge eat, etc.), and it covers a lot. I think the only way you really get a healthy diet is to gradually learn about it all, understand your weak areas, and work through and apply all the principles. In other words, it's a process of getting good at eating.
Nevertheless, I will share some "tricks" from my journey over the last few years:
(1) Accept that if a bad diet has been a lifelong pattern for you, then fixing it is going to be a ground-up reboot of how you eat. Think about it in terms of opening your mind, questioning everything, and re-learning everything.
(2) Pay attention to what you eat. Read nutrition labels, and maybe ingredients but that's less important. You don't have to do this forever because eventually you will internalize it. You will learn a lot. You will get a sense of what to favor and what to generally avoid. Having that sense will make a difference. (I used to think of "junk food" as a term that killjoys used to demonize enjoying food, but after reading labels, I finally understood for myself that some foods have a bunch of calories but little nutritive value. If these dominate your diet, you are going to have problems just like you'll have budget problems if you spend lots of money on frivolous crap.)
(3) Learn to love healthy foods. You might be tempted to read that as "accept the unfortunate fact that you're never going to enjoy eating again", but that's not what I mean. What I'm talking about is more like Pavlovian conditioning. We all have moments where we didn't have a chance to eat, we end up really hungry, and when we finally do eat something, it feels wonderful. Eat healthy foods for long enough, and these moments will build up and reprogram what you crave. One day your dinner will be delayed, and you'll find yourself fantasizing about the big salad, a huge pile of vegetables, and lean meat that you have planned for tonight's dinner. Aside from condition, an important part of learning to love healthy foods is trying lots and lots of them. Maybe you only like 1 in 5, but if you try 100 or more of them, you'll find something.
(4) Try to evaluate foods on two axes: like/dislike and healthy/unhealthy. This gives 4 quadrants. You should basically never eat anything from the dislike/unhealthy quadrant. And rarely does it make sense to eat a neutral/unhealthy food. Foods in the like/unhealthy quadrant are OK but not often. Foods in the dislike/healthy quadrant are fine but definitely seek out and prefer things in the like/healthy quadrant.
(5) Society is against you. Not on purpose, but look at obesity statistics (or just look around you) and see that most people don't eat healthy. Get used to the idea that you will be setting your own rules, which means there is a social aspect to this. Many restaurant menus will be packed with unhealthy options. People will pressure you to eat dessert with them. Not because they want you to fail but because it's the only way of eating that they know. It's best if you can do your own thing without calling attention to yourself. Graciousness makes things easier. I find positivity helps when you turn someone down. For example, if a co-worker brings donuts, I might say something like, "Wow! That was really nice of you! Unfortunately, I already ate breakfast and I don't need anything else, but those do look good!" Try to make them feel their gesture was genuinely appreciated.