Find a solid routine like the strong lifts 5x5 mentioned above, and eat. Most of the people complaining they can't gain and they eat enough most of time don't enough. If you want to be say 200lbs, eat like you are 200lbs.
I do Installs and flash tuning in South Florida
-2008 335i SGM on Terra 6mt
-1997 Jetta TDi 16v ABA Pt6266
-1997 M3 Pt6766