Building muscle mass isn't so easy to achieve and it wouldn't come very fast as you probably think. It is not achievable, not without using steroids. Building lean muscle takes some time and you can't gain 50 pounds of muscle mass in one month, you can only gain body weight like that, and that means mostly body fat.
First, you have to remember that if you want to build muscle you have to get stronger. To get stronger you have to lift heavier weights in the range of 6 - 10 reps and you will also get bigger. For totally new lifters I advise that they should first start with bodyweight exercises and gain some strength and muscle growth before they start lifting heavy weights. Pull Ups and Dips are very good Muscle Mass Builders, those exercises should always be in your workout routine.
Also, besides getting stronger, to gain more muscle mass you have to eat more healthy foods every day, you have to consume a lot of proteins, carbs, and fats. For tips on which foods to eat and what amount of calories to eat you can read my other posts:
Together with the workout routine and diet plan for muscle growth, supplements can be very helpful for building muscle mass. They are not necessary but will make muscle building easier and faster.
Read more about supplements for muscle building:
Compound exercises which work more muscle groups are best for muscle building. Your muscles won't grow with a workout that is consisted of machine and isolation exercises. You need to boost your testosterone levels and increase muscle growth with hard and heavy loaded workouts. Performing these exercises in the rep range 8 - 12 is the best for muscle hypertrophy.
Read which exercises are the best muscle builders:
Now, all that you need is good Mass-Building Workout Routine and your muscles will start growing and you will see results after the first month of training. When it comes to muscle building workout routines, you can choose between Full-Body and Body Split workouts. I recommend you to try one of the full-body workout programs if you are a beginner:
One of the most famous workout programs in Bodybuilding is Reg Park's 5x5 Workout Routine. Many bodybuilders like Arnold Schwarzenegger used this workout to gain more muscle mass and strength. It is very recommended for beginners and it is also a full-body workout program:
Now, you have all information about Gaining Muscle Mass, the last tip for you is to be persistent and don't just waste your precious time in the gym. It is better to train one hour hard than two hours of slacking. Muscles won't come so fast, but if it is that easy to build muscle, everyone would do it and it wouldn't be some great thing. Just keep training hard and eat a lot of healthy foods and muscles will come in some time.