100 calories of candy/sweets and 100 calories of an apple will work very differently inside your body. The apple calories carry vitamins, minerals, fiber and energy that your body actually needs so it gets used up simply by moving about your day. Sweets on the other hand, have basically no nutritional value and you get the harmful effects of sugar and possible weight gain.
Compare this -
250 calories of a fast food burger (this is one of the small plain burgers, not the quarter pounder kind) has 10 grams of fat with 3 of those saturated fat (not the good kind), 375 mg of sodium, 30 carbs, only 15 grams of protein, and even contains some sugar. There is almost no health and only harmful ingredients that send your body and health backwards.
BUT with that same 250 calories you can eat a 4oz grilled chicken breast (larger than the burger patty at fasts food) and broccoli (or any green veggie you prefer) and you get only 4.5 grams of fat (the good kind), 10 carbs, 51 sodium (the natural kind in food) and a whopping 38 grams of protein, not to mention all the vital vitamins and minerals your body needs to transform and promote super health. This is the kind of food that is completely used up to move, workout and LIVE. Nothing to store = fat loss/weight loss
So although you should have a general idea of calories and nutritional breakdown in food, if most of what you eat is whole and healthy you'll never need to worry about calories again.
Protein + good carbs (the starchy and produce kind) + good fat + fiber + at every meal + consistent exercise = Rock'n body and super health.
Make WHAT you eat about quality food not calories.