Introduction: Many young bodybuilders and fitness people start out underweight and are naturally skinny, however that should not let that deteriorate them!
How To Gain Muscle: To gain muscle size and mass one should be in a caloric surplus and follow a weight-training program. One should also have enough rest while they train intensely. Remember one does not grow in the gym, they grow when they go home eat, rest and recover!
Supplementation: Supplements should be used for the one looking to gain quality muscle mass and size. The supplements that I would recommend are protein powders and weight gainers, however remember that real FOOD beats any supplement out there. Supplements should only supplement your diet, if you feel your not getting enough calories or protein in your diet, then include a protein powder or weight gainer in your diet.
Do Not Stuff Yourself: By this I mean do not eat one or two big meals a day, power-lifters do that, not bodybuilders. Remember to eat every 3-4 hours and eat at least six meals during your day. The meals that you eat should include a protein source, carbohydrate source and fat source.
A good meal plan looks something like this!
Meal 1 (9:00am): Six eggs scrambled with a cup of porridge or oatmeal
Meal 2 (12:00pm): A meaty sandwich with a cup of yogurt and cup of milk
Meal 3 (3:00pm): A meal replacement shake such as a protein shake, carbohydrate shake or a mass gainer
Meal 4 (6:00pm): Here you will eat your dinner, now this might include three chicken breasts and two cups of rice, with a cup of vegetables
Meal 5 (9:00pm): A meal replacement shake such a protein shake, carbohydrate shake or a mass gainer
Meal 6 (10:00pm) A bowl of oatmeal and a glass of milk