How can I gain weight?

Ok, I am 100 lbs. And, I think its nasty. But, no matter what I eat or how much I eat...NOTHING! It seems impossible to gain weight. I asked my dr. recently what he could do about it...he said nothing, that im healthy. But, I look awful!!! Does anyone know any way or any food that will make me gain weight? Please help me...i'm willing to try anything at this point. ):