My father, mother and most of the rest of my direct blood relatives have always been really thin but I would really like to try to gain some weight because I am horribly underweight at around 110 at 5'9 in height. Any good tips to gain weight?

Protein and Carbs.. make sure to eat 3 meals a day with a light snack somewhere in there.. DON'T miss meals..

thats about all I know really..  But hope it helps