How to gain a little weight?

Try eating more fast foods and increase your intake of sodas.

you might want to increase your carbs and fat intake. but make sure you go for the healthy unsaturated fats because saturated fats could be detrimental to your cholesterol and your heart.

I suggest you don't eat more fast food and oily foods its not good for health. Try to healthy fat. Eat banana, milk, dry fruits and normal foods. Gym is good ideas. Add egg, chicken, meat in your diet, its healthy and good source of protein.