He is right about the celery thing, and actually there are tons of fruit and vegetables that have negative calories - I can do a search and get some for you.
hot chili peppers
Those are all negative calorie foods, so eat a lot of those. And if you have to put something on them, low fat sour cream (I hate it, but some like it) would probably be okay on celery and stuff, dont go overboard. It all depends on portions and sugars. Sugar is okay every once in a while, try to start cutting what sugars you eat by a little bit, then go down the next day and the next etc. until you are very rarely eating it. Instead of regular sodas, drink diet - most of them taste almost the same. Diet dr. pepper, Diet pepsi or coke, diet sprite - they all taste very similar to the sugary version. It may taste weird at first but if you keep drinking those instead, it will taste normal to you. And yes WATER, water is key - DRINK LOTS! Drinking cold water will burn calories pretty well. Focus on your eating before you get all worried about excersize.