A journal entry is usually a chance for someone to express his inner thoughts, and your teacher obviously wants to hear what you think Napoleon might be thinking throughout Animal Farm, by George Orwell. In this case, you might enjoy imagining what Napoleon is thinking about during several key events of the novel. For example, what happened and what was Napoleon thinking at this moment:
[T]here was a terrible baying sound outside, and nine enormous dogs wearing brass-studded collars came bounding into the barn. They dashed straight for Snowball, who only sprang from his place just in time to escape their snapping jaws.
What is Napoleon thinking as he hides in the barn during the rebellion, how does he train the puppies to become his killer bodyguards, what is it like to live in the luxury of a farmhouse for the first time, how does he develop a plan to get rid of his primary competition, Snowball--these are all interesting things which we know must have happened but which we never actually read about in the novella.
Another approach to journal-writing is to simply record events which have happened. In this case, a pre-rebellion entry might include a kind of "day-in-the-life" description of life as a "normal" pig before everything changed. The same kind of entry might be effective at the end of the book, when Napoleon has simply become another farm owner. I do not know how much leeway you have to be creative, but it might also be interesting to read an imagined obituary of Napoleon's life.
As you ponder your choices, make a list of all the "big moments" of the novel and determine how Napoleon observed them, planned them, or reacted to them. Obviously, both of these approaches can be effective in telling Napoleon's story in Animal Farm, and perhaps a combination would be an interesting presentation of his life over the course of the novel.