Hello there,
If you want to lose weight, become fit, and stay fit and healthy, you NEED to eat in moderation! I'm glad you asked this question, because I think that inability to eat in moderation is one reason why a lot of Americans (I live in the U.S., not sure where you live) are overweight. Americans tend to consume large portions, especially when they eat at restaurants and dining facilities. But anyway, to answer your questions:
1.) In order to eat in moderation, you need to pay attention to your body. Always be aware of how hungry or full you are. If you are hungry, then by all means EAT! But the reason why some people are overweight/obese is because they eat even when they're full! If your body needs food and you feel hungry, then don't starve yourself! But once you are full, don't eat anymore! If you follow this tip and go by your hunger level every time, you won't overeat or consume too large portion sizes.
2.) Yes! It WILL help you lose weight! To only way to lose weight is to burn more calories than you consume! It's important to eat in moderation, even if you eat mostly healthy foods. I saw this one documentary of this one obese 5 year old kid who ate mostly healthy foods but ate them excessively. So as you can see, not only do you need to aim for healthy foods (such as fruits and vegetables), you need to control the portion size that you are consuming!
3.) Yes, it's OK to eat junk food AS LONG AS you do so in moderation. It's OK to eat McDonald's once every two weeks. But because junk food is by definition less healthy than healthy food, you have to REALLY control your junk food intake. You know how I told you to eat healthy food in moderation? Well, your level of junk food consumption needs to be even more moderate!
I REALLY hope this helps! Good luck!