How can I gain weight on my arms and upper body?

I struggle with gaining weight, my body doesn't let me eat a lot and so I am very skinny... I am 16 and weigh 100 Lbs I workout every other day but it doesn't seem to add much weight at all. My arms are very skinny as well as my whole body I really want to put on some weight and "thicken" up a bit as I can not stand to even look at myself... Please can anyone help me?