If you are reading this today because of your intense desire for self knowledge and the need to take the inner journey, you will struggle to take on board the caution I've already given about devoting too much energy in the deep mine of your own self. Both are part of life but don't allow either of them to consume your life. Many published journals were written with a view to future publication. The author tries to capture the reality and essence of daily life, and perhaps to develop a philosophical insight into these events. Some journals are merely a diary.
Some journals are the working notes of people who are attempting difficult work.
Some journals are the notes written by people who are simply trying to educate themselves as they work and read and gather experience. The form your journal will take doesn't need a conscious decision. If you begin to write, preferably in an exercise book so the format is relatively unstructured, you will soon be writing something you find interesting and useful.