Nutrition, your bodybuilding success depends mostly on what you eat.
There are a lot of peoples who just think that they should train harder to gain more muscle,
but the truth is that in most cases you should take a look at what you're eating instead.
No matter how hard you train you won't see any noticeable differences fast unless you are eating right.
These are some good examples of what your food should contain:

Consume around 0.8 -1 g protein per pound of bodyweight every day.
Some good protein sources: skinless chicken breasts, tuna (not in oil)
Eat unprocessed or minimally processed food, no fast food.
Eat 6 , small, well balanced meals a day each with around 20% fat 35% protein and 45% carbs.
Eat lots of vegetables, preferably fresh.
Eat omega-3s daily, flax or salmon are great sources.
Eat whole grains and no simple carbs like sugar, white flour, or alcohol.