Write down what you eat on an average day. Google the nutritional values, add up all the calories. Now add another 500 calories. Now eat that many calories EVERY day. Weigh yourself first thing every Monday morning. If your weight isn't going up, add in more calories.
Btw, if you're naturally skinny then you're not gonna look good for gaining weight. You'll probably still have skinny arms and legs and just get a belly, not a good look. Lifting weights = weight gain that actually looks good.