Diet and muscle mass. You'll gain weight if you're consuming more calories than you're burning. Do not choose junk food. Make sure you're getting all your necessary nutrients in calorie rich food. Also, by weight lifting you can increase your muscle mass. If these changes aren't working after several weeks, see your doctor for a specialized plan.
Easy... I guess it depends on why you are doing that. For muscle mass, which is heavier than fat, you can work out with higher weights to build muscle or you can just gain weight to gain weight. To increase your body size, intake more calories than you burn off. Have a milkshake every night and keep a close eye on your weight.