A good first step is to picture a healthy plate. One half of the plate is for fruits and vegetables, and the other half is for proteins and healthy carb
Depends on the rest of the dinner. If you add the egg to an otherwise low-calorie meal to raise the protein content of your meal it may keep you satiated for longer and help you avoid snacking, thus helping you lose weight. But if you just add it to an already high-calorie meal it won´t do your weightloss any good. Eggs aren´t magical but they do contain some good nutrition you might use in a balanced diet.