It is unlikely that you would like to gain weight from body fat, so we will assume that you want to increase muscle mass. There is only one way - training in the gym and a properly selected diet.
You need to eat often - 4-6 times a day.
From harmful products it is better to refuse , despite the absence of problems with excess weight. Fast food, mayonnaise, juices with a lot of sugar will not help you gain mass.
Complex carbohydrates - any cereals, pasta - will provide energy for training, you can eat them at any time days.
Protein - lean meats, fish, seafood, mushrooms, cottage cheese, soy and legumes - this is the basis for your muscles, it should be enough. Based on the norm of 1.5-2 g of protein per 1 kg of your body weight per day.
Healthy fats will strengthen joints, blood vessels, heart, which is important during intense workouts. Take flaxseed oil, olive oil (can be added to salads), fish oil or fish oil.
Vegetables, fruits and berries can be eaten without restrictions.
Train with heavy weights in the gym, you should have the strength to do only 6-12 reps per set. It is better to schedule the program with a trainer who will take into account the peculiarities of health, shape, etc.
To gain muscle, not fat, you need physical activity and balanced nutrition and rest (everything is important!).
Eat better, like in a sanatorium, 5 times a day.
Each meal should contain 15-20 grams of quality protein (after exercise, 25 grams of protein is better).
Be sure to eat vegetables (especially from the green group) and drink water (about 30 g per 1 kg of body per day).
You can add good vitamin complexes.
Rice and pork. This is the classic diet of sumo wrestlers. Beef, fish, and chicken will not help here, namely pork, but not fried, but stewed, better baked.