My list is relevant for athletes and people who are hard to gain weight ;)
10 things which help gain weight:

1. Tell your body, that it should grow by doing heavy weights in gym. More load more gain. EASY.
2. Use sports supplements. Most of them do not work. But the really best are creatine and protein.
3. Eat more! Eat more often. Eat more food which contain more protein and carbohydrates.
4. Sleep at least 8 hours a day. And sleep at night.
5. Be positive, imagine your future body, visualize it.
6. Use steroids. Actually not a best advise, but it works great and its true )
7. Rest more. Your body needs time to recover, its very important.
8. Do not do cardio exercises, they only make your weight decrease.
9. AGAIN. Eat more. I see you not eating right now. Go and take some food homie.
10. Don't do stimulant drugs like amphetamine or cocaine or smth like that.