First of all, do not diet. Diets only last for a short time. However, you should develop healthy eating habits. I have been slim all of my life and in good shape. Currently, I am training for a marathon. Here is what works for me:
Do not eat processed food. That means junk food and fast food. That also means most things that come from a box, like mac & cheese, frozen dinners, that sort of thing. Once or twice a week, I cook a lot of food from scratch and then I freeze them to eat later in the week. That way, I don't cook every night because I am too busy. When you cook from scratch, you don't have to worry about too much sugar, salt, chemicals, and other unhealthy junk. It's cheaper, too!
Eat mostly vegetables and plants. There are so many different kinds of veggies in the world that you will be able to find some you like.
Do not eat anything that is white: white bread, white rice, white potatoes, white pasta. These are simple carbs that burn up fast in your system. They are mostly sugar and do not have any fiber. Your body will use it up too quick, spike your blood sugar too high, and you will feel hungry again an hour after eating. I eat brown rice, brown whole wheat breads, sweet potatoes, and no pasta (or you could try whole wheat pasta, but I don't like it). You want the fiber of these complex carbs because they help you feel full and they keep your blood sugar steady.
Eat protein at EVERY meal! You don't have to eat a lot of meat. I only eat meat about two or three times a week. You can mix brown rice & beans together for protein. I like that mixed with veggies, salsa, and cheese. A serving of meat (beef or chicken) should not be bigger than a deck of playing cards. Other protein sources I like are eggs and peanut butter. Protein helps you feel full and it stays with you longer so you don't feel hungry. It also helps you to build muscle.
Do not drink sugar! This means no soda or fruit juice. Okay, I may have one can of Coke a week. However, I treat it like a dessert because pop is really just liquid candy. Fruit juice and sweet tea are no better. If I do have some orange juice, I mix it with half water. Most people get WAY too many calories from stuff they drink. Sports drinks are awful, too! I run every day and I am training for a marathon, but I stay away from that crap.
Get regular exercise. You don't have to run and train for a marathon, but you should walk whenever you get the chance. Take stairs instead of the elevator. Take a 20-minute walk every day after dinner. Stuff like that.
Never skip a meal! Make breakfast or lunch your biggest meal of the day. You'll have fuel all day and won't feel like you're starving. If you're not starving, then you won't want to get junk. Eat a light dinner. There is an old saying: Eat breakfast like a king, lunch like a prince, and dinner like a pauper.
Have treats once in a while, but not every day. I still like Coke, candy bars, cookies, and ice cream. However, I never buy them at the grocery store and bring it in the house. I will only eat these treats if I am out with friends. I eat junk, but not every day.
Don't expect to change overnight. Just do a little bit more each day. If you fall off the wagon one day, don't beat yourself up. But, don't give up either. Just do better the next day. Soon, you will feel better and look better and it will just be how you live. You won't have to think so hard about it.
I hope this helps!! Good luck!!