How to become fat

I am Asanka. I have tried to become fat but I couldn't. I have tried some medications(pills) and got fat in a short time but when I finished taking medications it begin to loss weight in huge amount and I am back to normal again What should I eat in particular so that I become fat in a short time.

I am a 27 year old boy. My height is 5.3 inch and my weight is 45kg. As far as I know I feel hungry in the mornings. I am really worried and disappointed in this matter. Please help

Asanka