How can I gain weight the healthy way?

By eating A LOT!!!!! Don't eat crap foods, eat healthy foods like fruits and vegetables. Eat lean meats with lots of protein in it. Supplements are ok, but you have to be careful what you are taking. Some weight gainer is good, but you will want to add some kind of fitness regimen so the weight stays as muscle and not as fat.

Hello Poyner ! Thanks for your valuable suggestion. I appreciating these all things. But i want to inform you i don't have any bad habits like smoke, drinking etc. Apart from that i am lean from my childhood.