The main thing is to create a calorie surplus, then the mass will begin to gain. Read more about this powerfully here - https://workout.su/forum_thread/5579-den-8-balans-kalorij-skolko-kalorij-v-den-nuzhno
First of all, analyze your nutrition, remove excess and add proteins. For muscle growth, you can choose a scheme: 4-5 grams of carbohydrates per 1 kg of body weight, 1.5-2 grams of protein per 1 kg of body weight, and 0.5 grams of fat per 1 kg of body weight. Go to a fitness club and do basic exercises for all muscle groups.