Try to avoid all prepackaged food. There is very little food that comes in this form that is healthy at all. They also include high amounts of preservatives and are typically junk food in the first place. Normally you will be able to find a healthier alternative that fits your life better anyway.
You can imrove the nutrition levels of a meal by replace pasta, bread, and other carbohydrates with vegetables. Instead of having pasta with sauce and chicken, try swapping out the pasta with steamed broccoli or lettuce. Instead of having that hamburger in a bun, try wrapping it in lettuce.
To design the most nutritious diet possible, try to only put things into your body that your body can actually use. Fresh fruits, vegetables and meats are best for your body because they are comprised entirely of nutrients that your body can break down and use. Meanwhile, some processed foods and drinks may have low calories or fat, but they also have low nutritional value.