Well, if oyu are fairly active and would like to gain mostly muscle, focus on proteins like nuts and meats. Building muscle takes a long time, but if your focused you could probably put on 15-20 pounds of mostly muscle on a high protein diet within 6 months or so. However, eat other things, or oyur body will not synthesize the proteins nd oyu will actually lose weight.
If you would like to gain weight and dont mind it being mainly in the area of body fat, eat lots of carbohydrates. Breads and pastas for sure with every meal, and whole grains and grains too. Try not too eat too much sugars, because then you wont bea able to control your weight gain and it will be very unhealthy. In highschool i had to gain 20 pounds in 5 months, and all i did was eat two smaller carbo filled lunches as compared to one varied one. So i would eat a sandhiwhich at 11 and then a pasta salad at 3. Throwing in an extra meal of carbs will definetly speedily put on weigth, but if you want muscle, try proteins.
Life long athlete