Full hearty meals. Exercise (build muscle). Also fast food but that's not a healthy way to gain weight.

There are supplements that can help you gain weight

If you want to look good, weightlifting. If not just eat more.

Eat a lot of meat and beans and bread.
A lot of protein

Mcdonalds. But seriously, excercise

Eat in the middle of the night. Eat chocolate, snacks, chips, cookies, ice cream, while crying in the middle of the night in front of your television. And then, eat a lot of carbohydrate such as white bread, rice, potatoes. You'll gain weight in no time if you follow my advice.