Eat more. I know it sounds simple but that's what you have to do. Make sure to not let more than a couple hours go by without eating whether your hungry or not. Stick with calorie dense foods. Also, protein shakes can help you get more calories in if you can't stomach so much solid food.
for food isn't the basic calculation like 500 calories more than what your daily intake should be ? ( you need to determine the latter of course) . if you're a hard gainer, your diet must allow enough carbs as well as protein and fat.
then you need to determine proper workouts for size gains.
If you are a hard gainer and trying to put on weight, an easy and cheap way is to drink as much as a gallon of milk throughout the day (every day) in addition to your 5 or 6 meals. This is difficult for many people but is a great way to just add a lot of calories and put on mass. Now is a good time of year to do this and then in the Spring start a more serious diet plan to lean up for Summer.