Keep hydrated and your best bet would be to add creatine. Stay away from hormonal products. Most people end up relying on hormonals for years to get gains, and still have little in terms of true gains at that point. Get hypercaloric, and again, add in some creatine, if anything.
Creatine will help boost strength and will help with adding weight to each lift. More weight in a given lift equals more calories burned. This means you need to eat more, which should translate to growth over time.
There are several versions of creatine out there, I'd look into creapure, a pure form of creatine monohydrate (AX German Creatine is a good choice). Magnesium Creatine Chelate is also good as well.