Nutrition plays an important role when you are working hard to change your body composition. Not only are you building muscle, but you are working your body hard. Just as much emphasis needs to be placed on nutrition as the workout itself.
Hydration is a part of the nutrition. Throughout the day, drink as much water as you can. There are some great guidelines to follow in the article here. Drink 2-3 cups prior to your workout, and 2-3 cups following, again you can find specific guidelines here. The general guideline of 8-10 cups per day remains true for an average day, and more if you are exercising or in the sun/heat.
During exercise it is also beneficial to drink a good energy or sports drink. Personally I have switched to the energy drink Pro from Visalus because it has less sugar. I find it gives me that extra energy, and helps with post workout muscle recovery. I drink a Go prior to my workout, then Pro during. For reference this comparison chart explains why I choose these products, as well as it gives me more energy with better recovery following the workout. Whatever you decide to use, the most important fact is: look after your nutrition if you want your body to build muscle.
The next tremendously important part of nutrition when building muscle especially is the food that you give your body. Protein is absolutely essential. Again I use Visalus, but you can use another product if you like, or try to get enough protein in your diet with food. I tried adjusting my diet first but found fasting muscle growth when I started using the Visalus shakes, so personally I am sticking with it.
