How to gain weight in a quick time?

I find too difficult to add weight. I'm 21 yrs old male with 172 cms height and 58 kg weiht. Now i'm losing weight too, I dont know why, but i lost some of my weight. I can feel the difference. Even i'm having enough food for 4 times a day i couldnt add weight. Pls suggest me some tablets or something which will make me to eat more and to gain weight. I'm seriously vexed here. Pls help.. :(