Hello,
This is Dr. Klarida Papaqako answering your question.
Some things that may work to help you gain weight ans form a musculous, healthy body are:
1) Eat a healthy diet, with a variety of food, especially the foods who have lots of fibers (beans, fruits, vegetables),
proteins (dairy, meats, eggs) and drink plenty of water.
2) Eat 3-4 times a day, with snacks in between. Do it regularly, so your stomach will get used of the food slowly, and will accept larger amounts with time.
3) Exercise daily, start with 20 minutes at least 5 times a week, increasing gradually. Sometimes gym helps also, and there are exercises, who will help you gain muscular mas, that the gym instructors may recommend according your body stature.
4) Take multivitamins, according your age, who will help sometimes with the appetite.
Hope this helps.
Regards.