Metabolism in Humans as well as all biology is incredibly complex. In fact the battle for balance and order as far as energy use and storage in your body is raging right now. If you have interest in gaining increased muscle size and or improving the abilities of those muscles you have your work cut out for you. If it was easy, we would all look like Arnold Schwarzenegger.
Adding lean body mass requires eating a surplus of calories, and working out appropriately to convert that surplus into increasing your muscle cell volume. Maintaining your current weight requires just eating the number of calories a day that your body will use.
Few people know that there are mathematical forumulas for how your body burns calories--using them, it is quite easy to tell just how much you should eat!
The first steps will calculate your Basal Metabolic Rate, or BMR. Your BMR is base number of calories your body needs each day to just keep you alive--breathing, heart beating, etc. Be honest. The results will depend on you providing good information. Treat this like a doctor visit, not your driver's license weight.