All the advise for putting on muscle mass so far seems on the money but we are neglecting two things outside of training which is subjective and eating properly, the good old training table. Stress can keep you from making gains so you must take supplements like iron and B complex according to your size and weight. Your protein intake is minimally 1 gram for each 2 1/2 pounds of body weight. You have to have it every 2-3 hours in increments of 25grams each serving of protein otherwise, you will have to but extra newspapers and take a seat on the throne more often. Don't try to take all that protein at once. Dehydration will definitely be in your future. The other vitamins are easy to remember like ABC DE, they're all in those packets you buy and try not to take too much, remember the throne room. To offset stress, you need rest and you need to relax because sometimes your lifestyle, especially in the beginning of your work life is very stressful. Without rest, proper rest and taking it easy when you are supposed to, you can kiss your gains good by.