To gain weight you need to eat more calories. Its really as simple as that but you can buy drinks from the Chemists that are high in calories .. Complan is one
Eat plenty of carbohydrates. Potatoes are a good source. Also try working out (lifting weights rather than aerobics, which may make you lose weight), as muscle is heavy.
Become a complete square eyes with nothing to do except wade through websites answering daft questions..oh! No do you mean u want to be fat or do you want to bulk up try a high carb diet as suggested with lots of workouts as well