HOW TO GAIN MUSCLE FAST
Gaining muscle is not as easy as gaining weight, for gaining muscle you'll have to eat enough to provide your body with enough fuel so that your muscles can grow fast. You'll also have to provide stimulation to your body through exercise.
How much stimulation is widely debated. With some advocating strength training, some hypertrophy and some a mix of both, it can be really difficult for someone new to exercise to decide which to follow.
In reality, if you aren't training for a bodybuilding competition, following either of these approaches will help you gain muscle at roughly the same rate. The more important factor would be to choose an approach that you enjoy more, look forward to doing and will do consistently.
Consistency is important when you're trying to gain muscle, if you only exercise one day a week, gaining muscle will be difficult. Try to stick to your exercise plan as much as you can, but always make time for other stuff in your life. And if you're tired, take an extra day off to rest.
Rest is the factor that many people who are new to exercise tend to neglect. If you're eating right and stimulating your muscles enough for muscle growth, but you're not resting enough, it will affect your results drastically. Always try to get at least 8 hours of sleep every night.