Ive seen a lot of people who just join a gym , hire a personal trainer and do the workouts prescribed by them without even knowing their bodytype and the diet which is required for muscle gain.
Basically there are three body types
People with a very slender and thin physique are generally classified as ectomorphs.
If you are skinny, have a thin waist, narrow bone structure and a slender body frame then you belong to this category.
Ectomorphs are true hardgainers and have a difficult time putting muscle
The good news is you dont put on fat either.
But then how do you add muscle ?
First you need to understand what is a maintenance caloric diet. When you eat the same amount of calories as you burn that is your maintenance caloric diet.
Ideally for for your calorie intake multiply your body weight by 15. For example if you weigh 200 pounds your calorie intake will be 3000 calories.
But to add muscle you must have another 500 calories in your diet. This way your body does not go into deficit when you workout.
Also you need to perform workouts with lesser repetitions (6-12) and more weights to increase the hypertrophy or the muscle volume. More repetitions will simply drain your glycogen reserves leaving you depelted.
This is a very gifted and aesthetic body type. Most of the bodybuilders from the golden era like Arnold, Franco, Frank Zane had this type of body.
They have a medium body frame and easily add muscle but also a little fat. So they have to keep a tab on their food intake.
The diet should be 200-300 calories more than your maintenance. Also make sure you dont add fat by eating junk.
This body type gains fat easily and find it very hard to lose weight. Most of their body content is fat and not muscle.
Endomorphs should have a diet 100-200 calories lesser than the maintenance.
Also, the workouts should be ones which focus on endurance. The ideal rep range for each set in a workout must be between 12-15.
So identify your body type first then put the above tips into practice.