While plain popcorn is a less fattening then certain other snacks, and the corn itself is not fattening, its fryed in oil! Why do you think potatoes are healthy and chips are fattening and considered junk food, its the oil.
You should not need to snack on things like that! if you are so hungry, have a fruit.
If you have so much junk food and sweets, cutting them out could on its own prove enough to loose weight but the excersize you do is very little, you do not say how often you do it but a minimum of 2 hours a week would be very beneficial, it does more htne just loose weight, it makes you healthy, it helps your mind aswell. Im at a good weight i do 2-4 hours of excersize a week (right now slightly less due to revision, thou saying that i just got back from playing football for 3 hours, i did the same for 2 hours on sunday and did weight training on tuesday so have done 6 hours since sunday) not because i need to loose weight but because i want to stay in good shape, at my peak i can cycle 90km in 5 and a half hours, offroad.