[Arkan]

To gain 21 Lbs, you need to seriously alter your diet. Diet is the single most important thing in fitness. You need to eat a protein enriched diet consisting of skinless chicken, tuna, fish, eggs, and a protein suppliment drink. You need to figure out how much calories you're burning now and eat more than that. Eating 5 - 6 small meals a day is key.

And weight training is vital. You need to go heavy with low reps. Also, squatting is the single most important excersize when trying to put on weight. When doing squats, you're working out most of your body. You can cut the cardio down a bit also.