You're going to want to read about "resting metabolic rate". A decent primer can be found at this link
. Your resting metabolic rate is the number of calories per day your body burns at rest. Obviously you are not at rest, but it's the most useful number to understand the kind of input you're going to require to gain weight.
At the link above, I put in your age/weight/height/gender and the calculator estimated your RMR is 1498 calories. This number is a simplistic estimate as a health practitioner would come to this number using a caliper to determine body fat percentage so they could estimate your lean body mass (think of this as the amount you would weigh if you were 3% body fat like Arnold in his prime).
Here's another link
that provides an estimate for your lean body mass given a few measurements. If I remember correctly, I believe you can multiply your lean body mass by ~13.9 for a decent RMR calculation. One factor that the internet can't estimate is GENETICS, which is a significant part of the real life number.
To gain 1 pound, you need to input 3500 calories (input=cheeseburgers) more than you burn. Calories you burn per day = (RMR + calories you burn from stuff like breathing and walking + Genetics). Honestly, I would not worry! You're still developing. Being aware and making an effort to adjust your input to reach a desired weight is a good habit as long as you're not beating yourself up about it. Try to keep the grams of protein somewhere near the grams of carbohydrate, and don't worry too much about the fat given your goal (within reason...).
If you want to increase your appetite and put on muscle, work your big muscle groups at the gym such as your back and legs (rows & lat pull downs, bent over rows, squats, leg extensions, calf raises, and even your chest). In addition to increasing appetite—this will also increase testosterone, burn more calories, and may actually spike your metabolism resulting in...wait for it...an increase in your RMR multiplier. Vicious cycle! Good Luck!
EDIT: The conclusion in the previous paragraph about RMR being affected by activity is directly refuted in the last paragraph of the RMR article I linked. There I go again, thinking old wives' tales = data.