How can i lose weight? my weight is 77 kg at age of 18

Eat fewer calories!: Helpful weight loss diets have natural foods that take time to chew but contain few calories. Examples include carrots, celery, and iceberg lettuce (little/no salad dressing). When shopping for fruits, buy small ones no bigger than a tennis ball, because fruits have sugar. Drinks without calories include water and green tea. A calorie is a calorie, but surely avoid sugar & simple starch calories.

Nutrient dense foods: Eat foods that are nutrient dense (like meat, fish, poultry, eggs, cheese, low starch vegatables and water) avoid foods that are not nutrient dense (like starches and sugars, including fruit) nutrient dense foods will increase your basal metabolic rate. On the other hand, starches and sugars impair thyroid metabolism via affects on your liver (which helps activate thyroid hormone).