How can I..........???????? ?

Ok, I'm 15 years old, 100 lbs even, and, 5'1. No matter what I do, I can not gain weight, I asked my dr. if it was unhealthy and if I could get a medicine for it, he said no. I am healthy. But, I disagree. I wish I could gain at least 10 lbs? I already eat like crazy, im always hungry. But, nothing is ever enough?! Please help me. Do I need some kind of vitamin? Any suggestions? Thanks loveys(: