To gain weight in a healthy way requires eating calorie rich nutritious food coupled with exercise, eating multiple small meals encourages your body to use the calories to build muscle.
Drink milk each day.
Take less tea.
Eat bananas and fruits you like, avoid junk food.
Drink fruit juices, which give you vitamins and minerals.
In order to gain weight do not indulge in fattening fast foods which is the wrong route to take if you want to remain healthy.
The focus should be on eating nutritious foods from a variety of sources.
Some programs recommend expensive weight gain medicines but these supplements are not necessary so long as you are eating a well-balanced diet that includes more calories per day than you burn.
Remember "early to bed and early to rise" is key to health.