Hello,
Increase your caloric intake, take in more than you exercise off. Also, there are "healthy" smoothies you can drink that you can get at a health food store, these are usually mixable powders that you mix with milk and fruits, these will help to put the weight on as well.
You need to eat more calories than the amount that you burn.
Every calorie that you eat above that what you burn will eventually be converted into fat and stored in your body. (regardless if these calories come from protein, fat, carbohydrates or alcohol)
1 kilogram of fat is about 10.000 kCalories so if you daily eat 500kCals more than you burn you will gain a kilo every 3 weeks.
remarks:
-you will not gain much energy with a few extra kilo's of fat unless you are currently underweight.
-it is very important for your heath to get the extra calories from a balanced diet with the right mix of proteins, fat and carbohydrates