Skinny guy - gaining weight. what's the best regular food item to eat? any supplements that can slow down the metabolism?

Complex carbohydrate: Complex carbohydrates such as sweet potatoes, whole grains, beans and legumes - provide starch, stored as glycogen which is available to you for your workouts and for your muscles- so when you do resistance training to build up muscle mass- you will have the energy to do so. Building muscle is probably the best way to gain healthy weight. GOOD LUCK