I think that there are different approaches that can be taken with a journal entry. If there are specifics that have to be followed, I would pay attention to these before anything else. The entry might be designed to extract a response from you about what has been read in the first chapter. If this is so, then it is fairly open ended. You could write about how Lewis has established the opening, or exposition, of the novel. Perhaps, another strand of thought that can be explored is your impressions of the characters in the initial chapter. Discussing their traits or their early motivations might be something about which writing can be done. Discussing the stepping through the wardrobe might be an entire response in its own right. It is not lost on the reader that the setting is Britain during World War II. The presence of evil in the world can only be escaped through a wardrobe. Discussing the implications of this in a response could be quite fascinating and worthwhile.