Two things, first at your age you are what is called a hard gainer meaning it take a lot for you to gain weight or muscle mass so I would say find books or sites for hard gainers...when I was your age about 16 years ago, god I am old, I read a magazine called Flex it was a weight lifter body builder mag with stuff for everyone. I have looked at recent ones and they still seem to be up on something for everyone.
Secodly, you may be a light guy for life and never really get above 130-145 lbs. don't worry its your natural weight class so I would say look at WEC and the BW champ Migeul Torres, Chase Beebe, Faber, Manny Tapia, Garcia, and Pulver for inspiration as they are all great fighters at lighter weight classes and stick to it. Finally, be happy with what you got and make it work to your advantage, never stick needles or drugs in your body to get that quick fix you will probably get bigger but as I said look at what Torres, Beebe, Pulver have been able to do with their "small" size.
Its not about the size of the dog in the fight but the size of the fight in the dog.