An inference is an "educated guess" based on the context of what you are studying or reading. For instance, if I told you that I took my little brother to get his learner's permit today, you might make the inference that he is around 15 or 16 years old. This is because it is general knowledge that you can get a learner's permit around that age. You would also be able to easily make the assumption that I am over 16 years old, since I said that he is my "little" brother, and therefore must be older.
For your story, I would suggest outlining a list of what you know about the story, especially what has happened in the end that you know for sure. Then you will be able to make an educated guess about what you are unsure of in the story you are reading.