Like Zyg Says but . . .
Eat like a freaking horse. At your age your metabolism is probably very high and you have a hard time keeping weight. Setup a good eating regimen and do not over train. I guarantee you will see some results. My brother in law is the baby of his family. He is twenty and I have been helping him with his eating habits and lifting. He works out a home with a cheesy weight set and he is gaining very nicely. The punk is going to be bigger than me. Nothing like Natural Test to build muscle . . . .