You have to teach it what to eat, what not to eat, where to drink, how to cast spells, how to help the villagers, where to go the toilet, etc - it's a long, sometimes tedious, often amusing process. As such I was a little worried about leaving my computer running while I went to work, but I was quite eager for my creature to grow bigger, so I left him to his own devices hoping he'd manage not to repeatedly starve to death. I came back to discover my creature had eaten over 150 animals as well as a fair few villagers. I was not impressed. This comes from both meat and vegetables, although the former is higher in quality and more digestible than the latter.
Carbohydrates usually come from plants and are divided into two categories - simple and complex.
Fat is the main source of energy, but also provides a number of fat soluble vitamins. Fat helps provide healthy skin and contains acids which help regulate metabolism. Water is, well, water. Provides energy and inherently gives additional vitamins. Used up over time and through exertion.