1) You can't spot reduce fat - eg you can't do ab exercises to take belly fat off. The body doesn't choose where to take fat off.
2) To lose weight you need to burn more calories than you take in.
3) Diet is probably the biggest thing, but working out will help.
To lower calories don't eat sweets, or don't drink fizzy drinks. Try to not eat junk food and cook your own food.
For exercise, try lifting weights as when you do, your body continues to burn calories even AFTER you have finished working out, making it very effective for losing weight.