The secret to what skinny people eat to stay slim is really different for everyone. Some people do really well on a diet that's higher on protein and vegetables and they can't eat any of the complex carbohydrate,s even whole grains.
Some people do really well on a plant based diet that's lower in calories and they only use plant-based proteins and they stay slim. Really it's about finding out what works best for you. Some of the obvious answers to look at are, are there any food sensitivities that you have that might be keeping on weight in a way that you can easily eliminate?
Look at, is your diet high in corn, or wheat or gluten, dairy, soy, even eggs? Those are some of the most common food allergens that keep your body in a state of being unhappy and keeping on extra weight, even when you try to lower your calorie intake.
So by removing any of those offending foods, your body can really start to relax and lose weight easier. Maybe you need to go to a doctor or an allergist to find out for sure if you have any food sensitivities.