Are you very active? If so your calories will be much higher than a person who just lifts passively. Start keeping a log of what you eat and how many kcalories you are burning. There is a great free nutrition software program that you can download from the USDA. Its called the USDA-SR 18. Google it.
Try using this figure:
20-25 Kcals/kg of bodyweight. - if you are overweight and want to
lean out
30-35 Kcals/kg of bodyweight. - to maintain your bodyweight
40-45 kcals/kg of bodyweight. - to gain bodyweight
You are young so I would err on the side of more kcalories. If your active, it takes alot to gain weight. Always, be very objective when you look in the mirror...these figures are for patients in a clinical setting, which is applicable in theory to training.