Eat good sources of proteins and fats, protein is an obvious one for muscle gain, many people who are new to body building presume fats are fatal to your body.
The truth, from extensive experience both with bodybuilding and molecular biology, fats heighten testosterone levels and promote general growth, as metabolism is fuel primarily by fat stores (therefore no fats store means no protein synthesis, which means no weight gain)
also make sure you are eating PLENTY of carbs, rice, pasta, potatoes.
Try to eat healthy!
Also a very good tip for naturally lowering metabolism (which will cause substantial weight gain) is to eat LESS meals but larger portions. This may seem obvious but the emphasis on 'less meals' is the key to weight gain. Read my article on eating more for weight loss if you wish to understand the concept, its essentially the opposite. I may write an article soon purely for this question!
Hope this helps!