You are a pure vegetarian and you need to eat healthy with the right combination for protein, calcium, and nutrients. At 15 years old you need appropriately 1400 calories per day. So if you eat 1800 calories, you may gain weight slowly but you may have a high metabolism and do not gain weight. You are young right now and as you age your body's metabolism will slow down and you will gain weight. Try not to worry about your weight at this age.