Weight loss occurs when you burn more calories than you consume. So, if you ate exactly the same but started walking daily you would slowly lose weight. Slowly that is, because going for a walk each day doesn't total up to a whole lot of calories, but it does make a difference.
You also mentioned eating less junk food. Cutting out some treats and replacing them with healthy, low calorie foods will make an additional difference. Look at the calorie totals in some of the junk foods you eat. They are very calorie dense. Now, combine eating less junk and walking and you should see a difference in weight loss. Still not as much weight loss as if you had a regular exercise regime of varying intensity cardio, strength training, a balanced and healthy dieting, proper water consumption.
Sarah