There are 3 major points to consider when trying to gain muscle mass. First is the type of workouts you are doing meaning strength training or hypertrophy training. Second you need to be in a caloric surplus meaning you need to eat more calories than you are burning throughout the day. Lastly you need to be eating a high amount of protein and carbs but spread out throughout the day because your body can only process so much food at a time and the rest will go out as waste.
I like to mix in strength and hypertrophy training by starting my workouts with a compound movement or two and ending with some type of isolation movement.
Some compounds movements would be the deadlift, barbell row, bench press, squat, etc… Hypertrophy movements would include isolation exercises like the bicep curl, single arm row, single leg leg press, etc… Try to incorporate 1-2 strength exercises with 3-4 hypertrophy exercises in your workouts.
What is a caloric surplus? A caloric surplus just means consuming more calories a day than you expend in a day. Its that simple just eat more than you burn. Your muscles need food real food to grow and with the training you should be doing then you are going to need a decent amount of food to build muscles lean muscles.
Foods to consume would be rice, chicken, meat, potatoes, veggies, etc… You can have more junk food or fast food but its not recommended because the goal is to build lean muscle mass instead of getting fat.
Protein is essential for muscle growth but you body can only absorb a certain amount at a time therefore it is recommended to space out your protein consumption throughout the day to maximize the intake. One way to add more protein into your diet is by snaking on protein bars during the week or adding in a protein shake into your day. Most people consume a protein shake after their workout which is good if you can’t get home or get food into you within an hour after a workout. If you can get home within an hour just eat real food and save the protein shake or bar for when your hungry before bed.