How can I gain muscle mass effectively ?

These three facts are most important for your success:

Training + nutrition + regeneration

1. Training: an intense workout helps your muscles to grow faster. It is important to train regularly and increase your training load progressively .

2. Nutrition: your muscles can only grow when you eat more calories then your body actually needs.
300 -500 Kcal / day is sufficient. However, make sure you have enough protein included in your daily meals.

3. Regeneration: Your muscles do not grow during exercise but in the days after. If your recovery time is too short, your training stagnates. Your muscles do not have enough time to grow.