Please help, I’m very skinny and I can’t gain any weight!

Patient: Some people suffer from being overweight, but i suffer from the opposite of it. I’m an underweight 15 soon to be 16 year old guy. I weight 110 pounds and i don’t find this normal at all. I try eating fat food, but this is not working out at all, since i’m using the bathroom every day and I don’t get hungry at all. I’m getting very upset, cause i can even see my ribs. Please help me understand my problem. I don’t want to be so skinny!