For you to understand how to gain muscle as fast as possible, you will first need to understand what circumstances lead your body to actually want to produce muscle. I’ll make it extremely simple.
Your body will only produce muscle mass if it has excess calories to do so.
By excess calories I mean that your body needs to have access to more energy than what it requires to sustain itself. To determine exactly how many calories your own body needs, go through this piece of info over here.
Let us say, for example, that you’ve determined your body requires 2,000 calories per day. For you to be able to gain muscle mass, you will have to eat more than these 2,000 calories. If you don’t, your body will simply be too busy trying to barely make ends meet for itself, energy wise. However, if you provide it with more energy than what it needs, say 2,300 calories in total, it would have 300 extra calories that it can use for whatever is top priority for it right now. (More on these priorities in a moment.)
The lesson I want you to take away here is this: you cannot lose fat and gain muscle at the same time. That’s because, to lose fat, you need to eat at a calorie deficit (consume less energy than what the body burns), while to gain muscle mass, you need to eat at a calorie surplus (consume MORE energy than what the body burns). As you see, the two rule each other out altogether, so if your goal is to gain muscle as fast as possible then you will have to forget about losing fat for the time being.
Now, in the example above I said that if your body requires 2,000 calories a day to function properly, and you consume 2,300 calories, then you are giving the body a “bonus” 300 calories to work with. This is when weight lifting will come into play. If you work out enough times a week, your body will prioritize building muscle mass from those 300 extra calories it has, over anything else. If you were not to lift weights, however, the body would see no need to build muscle tissue, so the additional 300 cals. you are giving it would get stored away as fat. Please remember though that there is a limit to how much muscle your body can produce per week.
If you are a beginner weight lifter with no prior experience, and if you are not using any kind of steroids, you can expect to gain approximately 1 pound of pure, high quality muscle per week. To do so, you would need to eat at a calorie surplus of around 400 cals (meaning provide your body with 400 extra calories per day). This, however, is an estimate, and the actual number will vary slightly from person to person.
So if you’re looking to gain muscle as fast as possible, you will compose a diet with a ~400 calorie surplus, and lift weights 3 times a week. Remember though that this is all advice for beginners – if you are a somewhat more advanced lifter then things will be a little different, although not by much.
You’ll also need to make sure that your body gets enough protein to build all that extra muscle as quickly as possible. An adequate amount of protein for a male who lifts weight is 1 grams per pound of LBM. That shortcut stands for “Lean Body Mass”, which is the mass of your body without the fat. Quick example: say you are 200 pounds heavy, and you know your body fat percentage is 20%. This means that out of the 200 pounds you weigh, 20% is pure fat. This equals exactly 40 pounds of fat, which would put your LBM at 200 – 40 = 160 lbs. Therefore, in order to gain muscle mass as fast as possible, you’d have to consume 160 x 1 gram = 160 grams of protein each day, in addition to making sure your body has the “bonus” calories to build muscle from.
Finally, you should make sure you get enough dietary fats as well (yes, they are not evil and you need them in fact). That’s because fats help regulate your body’s hormone levels, giving you -among other things- a spike in testosterone levels. And as you might know, testosterone is the godfather of all hormones when it comes to gaining muscle – the more of it your body has (within healthy ranges), the faster you can expect to blast your muscles into space. As for the amount of fat you should consume, make sure it’s as close as possible to 0.4 grams of fat per pound of Lean Body Mass.
Summary: sustain a 400 calorie surplus diet, lift heavy 3 times a week (preferably using a bodybuilding routine with a solid track record), consume 1 gram of protein per day per pound of lean body mass, and 0.4 grams of dietary fat per day per pound of LBM. Finally, try to avoid alcohol, particularly on the day right before your workout. That’s because it will likely result in a less than optimal performance in the gym, which over time will slow down your muscle gaining progress – and that’s not what you want, is it?