There are many reasons to improve your eating habits: weight loss, weight gain, increase in nutrients. Everyone's reasons may differ on why they want to change for the better, but here are some basic guidelines on what has helped me before.
Organize your meal times
Eat meals and snacks at a set time. Your body will become much more organized and so will your meals. This might help you avoid feeling intensely hungry and resorting to binge eating something bad, fast and on the spot.
Give yourself cheat time
100% clean eating will result in a junk food relapse. Give yourself some wiggle room and room to breathe so you can continue the diet.
Cook for yourself
Experiment with the things you like and don't like. If you're used to eating out or having others cook, you might not know how wide your food palate is. Try different recipes and know what you prefer. A carrot or squash can have endless flavor options. Know what you like and work off of it.
Discover worldwide cuisines
Try eating foods from all over the world. Every cuisine has new styles of cooking and different healthy spices and ingredients. The more you try, the more you will know what you like and the wider your meal options.
Give it time
It takes time to get used to a new diet. Eventually, this temporary feel of healthy foods will become a lifestyle. It just takes a shift in habit and an adjustment for the taste buds.
Slowly alter sugar intake
Processed sugar should be eaten in small, monitored amounts. Eat all the fruit your heart wants, just avoid eating/drinking too much of it. But it's not easy to cut huge amounts of sugar from your diet immediately. It should be done slowly for the taste buds to adjust. Once they have adjusted, you will realize how much sweeter things are. You can try to limit desserts at first to one less per day, then lightly alter the amount of sugar in your coffee/tea. Slowly, your body will notice a difference and crave sugar in lower amounts.
Talk to yourself
Sometimes you find yourself craving foods that will really do damage, or you've already had your cheat meal. If you've hit a wall and don't know how to control yourself, just talk to yourself and ration. Before you grab that piece of cake or a big bag of chips pause and ask, "what will this do for me? Do I have better options? What would happen if I skipped this food?"
Brush your teeth earlier than what you're used to
Like many others, I prefer to snack at night. It makes sense because this is the alone and downtime we have, so it's nice to have the last moments of the day to share with food and some T.V. To stave off temptation best as possible, brush your teeth after you have eaten your last meal, or just an hour or two before bed. It's not fool proof because you can always just re-brush, but at least you have taken an extra precaution that will mentally prepare you to stop eating.
Don't stock up on junk food
Don't cut yourself off completely from junk food, everyone needs their little stash. Just make sure it's a minimal amount, if you don't have it at home, you will have to resort to better-snacking options.
The more things you eat from nature, the more even the nutrition you will intake. Foods with preservatives tend to have more calories and less nutritional value. There are always exceptions, but this is just a general rule of thumb.