I am just 20 year old guy and i start getting fat as soon as i stop gyming. what to do?

Eat right: Eat a healthy diet of fresh vegetables, fruits, and lean meat. Drink plenty of water..8-10 glasses a day. Eat 4-6 small meals a day so you are never starving. Stay away from processed sugar products...Cakes, pies, cookies, and candy. In fact eat as little sugar as possible. Limit your alcohol intake as well. If you eat this way and exercise moderately you should not gain weight.

Cut down on your foo: Cutting down on food/calories would be helpful.If you watch out what you eat and eat less of calorie rich foods will help to prevent rapid weight gain.High caloric foods are milk and milk products like icecream, yogurt and many of the sweets.If you avoid fried foods and animal fats like in meat, that will help. Increasing activities like extra walking will help also.