GAINING LEAN MASS
Everyone wants results quickly, but healthy muscle development takes time. A big increase in weight in a short space of time is more likely fat, it is therefore better to increase gradually, healthily, and over time.
To gain muscle, you need to take in more calories than you are expending to allow muscle to grow. This means you need to be smart about what you eat to prevent unnecessary fat gain while still providing enough calories for muscle growth. Be realistic - weight gain of a couple of kilos is the most you should be aiming for over a 2-3 month period.