I can't stop eating unhealthy foods?

hi, im 13 years old and im not really fat, but i would like to be ALOT healthier.. basically, if i go in a shop, i have to buy something like chocolate or coke or pringles.. i can't help it, and i can't eat salad or any vegetables i hate them all.. i just don't know what to do any more coz im getting fat and i want to eat healthy and exercise.. but i have no idea how to help myself from eating junk food.:'(.. please help?.. any suggestions would be very much appreciated.. thanks so much:). x