I'm assuming you mean writing a journal from the point of view of one of the characters. This is a popular activity over the novel, and it is a good one because it requires that you are very familiar with the story and you understand the character you choose to reflect in a journal. For instance, let's say you choose to write a journal from Scout's point of view. What are some of the most interesting, irritating, frightening, or significant experiences that happen to her? How does she feel about them or react to them? How does she feel about some of the other characters in the story, and why does she feel that way? What does she not understand? What does she come to understand by the end of the story? What does she learn about herself and other people? All of these questions can lead you into some good journal writing from Scout's point of view.
If you are supposed to write a personal journal from your reading of the novel, apply those same questions, but think about your own reactions. For instance, consider the conclusion of the novel when Scout meets Boo for the first time and walks him home. What is your reaction to that? How do you feel about the way Scout treated Boo? Think about Miss Maudie and how she treats Jem and Scout. What was your reaction to Miss Maudie's baking a special cake for Jem after Tom Robinson had been convicted? Also, you could write journal entries about your feelings toward the characters. Which ones do you like? Which ones do you dislike? Why?
Whether you are writing from a character's point of view or your own, journal writing is a collection of observations, thoughts, and personal feelings about people and what happens to them. Below you will find some eNotes links that will take you to material over the novel to give you some more ideas. Good luck!