What are some (healthy) things that i can eat to gain weight? (i am underweight)

Raw and organic: Most raw organic foods are very helpful. Grass fed beef, wild fish, uncooked organic fruits and vegetables all help. Avoid processed food. Eating to get fat is not useful. Eating to get healthy is.

Key is exercise: If you're skinny, and you want to gain healthy body mass, the key is less about eating a lot and more about getting back into that fitness-focused lifestyle you loved as a teen. A man needs no body fat; a woman only a small amount. Avoid sugary, salty, and greasy stuff regardless. Many elite track-and-field athletes are "underweight" by the numbers; you're a person, not your numerical weight.