Healthy Weight Gain

How can I gain healthy weight? I am 5'11" and only 120lbs. The rest of my family is pretty thin also, but I've been told that I should gain weight by a lot of people including doctors. I just can't seem to gain anything. I eat at every meal, I even snack... and it doesn't seem to work. Is there a way that I can gain healthy weight and still keep tone?

Exercising can make you gain weight. Try going to the gym regularly.