There are many different ways to start off a parable. In light of this, let me give you a few examples. 

First, you can start off by saying something like: "In a farm there were many animals. Some animals were good and friendly, like the sheep. Other animals were not so friendly, like the wild horse." If you start off generic like this, you can make each animal correspond to something. 

Here is another example but this time it is more specific. "There was once a foolish and wise man. The foolish man wasted his time, while the wise man did not waste his time but worked very hard." Based on this, you will need to make a lesson.