Weight loss goal needs help?

Hi, 4 days ago, i started exercising. I have been using a walk at home dvd that i really like. I generally do about half an hour to an hour. The thing is that i still live at home and i have noticed that my eating is kinda bad. Like i got home from school and ate banana bread and 2 gles of milk. Also, i really love sweets and i have to make some soon for a school bake sale. I don't know what to do! I can't buy the groceries, so i eat whats in the house. Will a bad diet inhibit me from losing weight after putting in hard work exercising? I feel so hopeless and its only day 4. I am 16, 5ft 6in and 200lbs. I am trying to lose 40lbs by summer.