Eat fresh vegies and fruits. Stay away from sugar and white flour. Eat raw nuts (chestnut, hazelnut, peanut, etc.).
And more important than all of these; eat olive oil as much as you can...
consume Milk
because Milk is a source of protein and carbohydrates and many other nutrients so good to gain weight. As many as 100 ml of milk approximately contains about 3.4 grams of protein and if we drink 2 glasses every day, we will consume 14gram protein, good enough not.