HI welcome to HCM. i had gone through your question i can understand your concerns. A healthy diet is the foundation of gaining weight. Pay attention to your health and food intake.
To increase your appetite have a glass of lime water before your meal.
have minimum 4 meals a day , gradually increase the quantity you eat every time
Drink 1 glass of milk twice a day.
breakfast is the most important meal of the day that should include both carbohydrates and
proteins like oats with milk.
increase your protein intake by taking milk,dairy products cheese,panner,dals,nuts,pulses,chicken,egg whites,
Lunch and dinner should include combination of carbohydrates proteins and healthy fats which include dal,roti,rice,sabzi, salad.
have 4 almonds per day
drink 3 litres water per day
exercises targeting bigger muscles like chest,back,thigh ,butts will be helpful. full body exercises like push ups,squats,pull ups,rows will be helpful.