My Dear Friend,
Instead of becoming fat you must aim to become FIT.
Gaining fat is bad for health in long term. Also the physical constitution is mostly determined by genetics or some people in general have a high metabolic rate.
So I suggest you must aim to become FIT not fat.
Do follow steps listed below, it will help you to become fit.
1. Do 21 Suryanamaskars in front of rising Sun every morning, you can learn suryanamaskar by watching videos on you tube, but please do not try to achieve too much too fast, do slow and steady and achieve your goal.
2. Start working out in gym, build muscle, once you gain muscle mass, it will increase your weigh.
3. Eat a diet that is rich in proteins that is egg white, fish, chicken, nuts, mushroom, tofu, brocolli ,paneer and protein powders.. Let me warn you here, please do not take any of the weigh gain formulas being sold over the TV or other media, it may contain very harmful steroids that can damage your health for long term.
4. Eat at least five fruits every day.
5. Take a calcium and a multivitamin capsule everyday.